hi, Thanks all for replying.
I do know abt rootsh. i have installed that.
i do get root access when i execute sudo gainroot.
but.......
i want to know if i want to set a password for root.

Below are the steps i follow :-
sudo gainoot
passwd
--- i enter password (suppose i enter the password as abcde)
- i reenter the password

till here everything works fine.

now if i close the terminal and open it and try to execute this comment
sudo su - (trying to login as root)
it prompts me to enter the password (if i enter the same password abcde)
it throws an error :-
sorry, try again.