What are the Risk of adsense niche sites?is it according to the google 's TOS.I saw several niche web site have been banned ..
I don't think niche has anything to do with it. What were the websites that got banned? The sites themselves probably violated the tos
Creating a site purely for the sake of putting adsense on it (IE. very little content with just keywords to create ads) is against google's TOS. But by definition a Niche site is nothing more than a site with a designated topic. There are people who create sites based upon that niche's potential revenue...but there is nothing wrong with that as long as the site contains original (IE non-copywrite) content.
Niche is what it is all about, so this could not be the problem. Mention some urls and I'm sure you get explanations / speculations why they were banned