If some sites in links page in comment section given link with rel=(external,no follow) Google consider for this link to back link. What is effect of rel=(external,no follow) please help me.
It basically tells Google not to pass pagerank to that site. It had to be done because of spammers. Many websites also use this for "pagerank sculpting" by nofollowing pages like contact us, privacy pilocy, etc thus allowing more pagerank to flow to content pages. Check out this YouTube Video (Matt Cutts): http://www.youtube.com/watch?v=x4UJS-LFRTU
Most webmasters use rel-nofollow option to eliminate spammers. It instructs not to count link as backlink.
I believe the correct syntac is rel=nofollow, so if what you see is "no follow", yes Google will pass the link juice and backlink counted. The rel=external is mostly javascript/css tag, for example if any given link has that tag, css will show an icon next to the link to indicate to the reader that is an external link or will open new tab if click.