This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
First of all, please add some indentation. It's not easy to read this way.
Your flag will remain false if it even one number is not a prime, because the only place you set it to true is at the very start of the method. You should set it to true for each number you want to test; just move the declaration inside the first for-loop.