I need to move my domain from mysite.com to mysite.com. This is a must,i have to do this. What is best practice - i am going to redirect all pages of old site to exactly same pages on the new site of course using 301 redirect. Any advice.Will this do any bad to my ranking?
Yes, this will certainly hurt your ranking, at least in the short-term. However, if you do it correctly things should recover within about 1-3 months, depending on the size of your site.
The change will likely fix itself fairly quickly. I had about 50,000 indexed pages on a site I had to move, and it took a few months. In your case, it shouldn't take nearly so long. Don't screw up the 301.