Check the below link this will help to set default os ubuntu. GrubHowto
Back Up Grub Settings
Copy this line into the terminal and press enter, it will back up your menu.lst and you can use the backup to restore your default settings.
sudo cp /boot/grub/menu.lst /boot/grub/menu.lst_backup
Open in a Text Editor
Edit the menu.lst file with one of the following commands (depending on what version of Ubuntu you're using):
gksudo gedit /boot/grub/menu.lst
kdesu kate /boot/grub/menu.lst
gksudo mousepad /boot/grub/menu.lst
Finding the Line
Find this line in the document that was just opened, it will read like this:
Set the default entry to the entry number NUM. Numbering starts from 0, and
the entry number 0 is the default if the command is not used.
Replace the 0 with the number on the startup list corresponding to the option you want, counting from 0. For example, if you have a list like this one and want to change to Windows XP Home:
title Ubuntu, kernel 2.6.15-27-amd64-generic
title Ubuntu, memtest86+
title Other operating systems:
title Microsoft Windows XP Home
...you would change the number to 3. It is important to note that each lines such as "Other Operating Systems" should be counted as well.
Alternatively you can replace the number with the word 'saved' which will make the default whichever entry was last used. Each entry that should be remembered must have the 'savedefault' keyword, this is the case for the normal Ubuntu entries, but not the recovery alternatives. WARNING: If you are using dmraid do not change this entry to 'saved' or your array will desync and will not let you boot your system.
Save the edited file