If you are running regularly update in your system,the old kernel left behind when ubuntu installs a new linux kernel.The old kernels leave in grub menu,so grub becomes longer and longer.
You can check which kernel you are running by this command:

uname -r

Then use this command to remove unused kernels as well as in grub menu:

sudo aptitude purge ~ilinux-image-.*\(\!`uname -r`\)

If you’d like a GUI,Ubuntu-Tweak is a good choice where you can remove them from Applications->Package Cleaner->Clean Kernel
Install Ubuntu-Tweak,see:http://ubuntuguide.net/installupgrade-latest-ubuntu-tweak-in-ubuntu9-109-048-108-04