How to reformat my iMac

Assuming you are trying to reformat your startup disk (probably the only disk you have on your iMac), then yes, you will need a system install DVD. Either the one that came with your computer or an operating system DVD that you purchased later.

--Ron

You need to use the original DVD that came with your computer. http://support.apple.com/kb/HT3910

Why do you need to reformat? It's not usually necessary to do so if you have an isuue.

To answer your question, yes you will need to run the installer disk.