I am planning to visit Iran for a conference that will takes place at Tehran. My questions is that if I am visiting Iran, can it harm my visit to U.S.
If you are from a country with no visa needed to go to the US (for example ESTA from europe), then you'll have to apply for one.
Also border control may be suspicious if you travel to Iran and go to US just after.
Possibly yes. You will come under extra scrutiny, and nobody can predict the outcome of that with any certainty.