i make a free blog on word press and work on it. after some time his page increase 1. i purchase domain for this blog. My domain page rank also increase or not?
If you shift your wordpress blog to your own domain - then that will not be having your current PR - it will take time to reach to that PR Hope it helps...
Just put the redirect from you blog to the newly purchased domain. Thare are several ways to put redirect like updating in .htaccess, html script or java script. you can expect the page rank shift within a month .
Your PR of course safe. Yes for the best result use redirect the new domain name to old one. Good luck.