https://help.ubuntu.com/community/UEFI
Introduction
The Extensible Firmware Interface (EFI) or its version 2.x variant, Unified EFI (UEFI) is a firmware type that is widespread on recent computers, especially those more recent than 2010. It is intended to replace the traditional BIOS firmware that is prevalent on earlier machines. This page provides information about installing and booting Ubuntu using UEFI, as well as about switching between UEFI mode and legacy BIOS mode using Ubuntu. 
Most modern computers support both UEFI mode and BIOS mode. Which boot mode you should use depends on several factors:
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Boot mode matching -- If you're dual-booting with another OS, the two OSes' boot modes should match. Most computers that ship with Windows 8 and later use UEFI to boot that OS, so this configuration dictates use of UEFI mode when installing and booting Ubuntu.
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Hardware support -- Some hardware devices work better in one mode or the other. (Usually BIOS mode is better supported.) This type of problem is becoming less common.
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Boot loader problems -- Sometimes a boot loader for one mode works better than a boot loader for another mode. BIOS-mode boot loaders are better tested and are therefore less likely to pose problems.
Note that these factors can conflict with one another. For instance, if you're dual-booting with a pre-installed Windows 8 but have problems getting a UEFI version of GRUB to work, you'll be in a bind. As a general rule, though, UEFI mode works better in dual-boot setups with pre-installed versions of Windows 8. If you're installing Ubuntu as the sole OS on a computer, either mode is likely to work, although BIOS mode is less likely to cause problems.
Installing Ubuntu in UEFI mode
The following sections describe how to install Ubuntu in UEFI mode, either because you're single-booting and want to try this boot mode or because you're dual-booting with another OS that's already installed in this mode.
Case when Ubuntu must be installed in UEFI mode
Having a PC with UEFI firmware does not mean that you need to install Ubuntu in UEFI mode. What is important is below:
- if the other systems (Windows Vista/7/8, GNU/Linux...) of your computer are installed in UEFI mode, then you must install Ubuntu in UEFI mode too.
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if the other systems (Windows, GNU/Linux...) of your computer are installed in Legacy (not-UEFI) mode, then you must install Ubuntu in Legacy mode too. Eg if your computer is old (<2010), is 32bits, or was sold with a pre-installed Windows XP.
- if Ubuntu is the only operating system on your computer, then it does not matter whether you install Ubuntu in UEFI mode or not.
General principles
To install Ubuntu in UEFI mode:
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Use a 64bit disk of Ubuntu. (Ubuntu32bit cannot be easily installed in UEFI mode. This is a problem if 32-bit UEFI is the only way your computer can boot, e.g. if you have a modern Intel Atom based laptop. In this case, you will need a complicated work-around.)
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In your firmware, disable QuickBoot/FastBoot and Intel Smart Response Technology (SRT). If you have Windows 8, also disable Fast Startup.
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You might want to use an EFI-only image to avoid troubles with mistakenly booting the image and installing Ubuntu in BIOS mode.
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Use a supported version of Ubuntu. Support for UEFI appeared in 11.10, but has become more reliable in next versions. Support for UEFI SecureBoot appeared in 12.10 and 12.04.2.
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Set up your firmware (BIOS) to boot the disk in UEFI mode (see the "Identifying if the computer boots the HDD in UEFI mode" paragraph below)
- Then:
- nothing special is required if you use the automatic installer of Ubuntu ("Install Ubuntu alongside others" or "Erase the disk and install Ubuntu"). Important: if you have a pre-installed Windows and you want to keep it, do not choose "Erase the disk and install Ubuntu".
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if you use the manual partitioning ("Something else"), the difference is that you will have to set the /boot/efi mount point to the UEFI partition. And if there was not any UEFI partition on your HDD, you first will have to create it (see the "Creating an UEFI partition" paragraph below).