The first round which confirmed 208 seats in the country's 290-seat legislature was held on March 14 with the conservative party winning 132 (over 70%) of seats. The current polls are expected to reinforce the conservative majority in the parliament.
A total of 164 candidates, including 69 conservatives and 41 reformists, will contest the remaining 82 seats.
Relations between Russia and Iran are expected to be unaffected by the outcome of the polls.
Iranian Supreme Leader Ayatollah Ali Khamenei, who earlier declared voting a "political and religious duty" that every Muslim should perform, was among the first to cast his ballot in Tehran.
Turnout is expected to be as high as in the first round at 60-65%. Iranian authorities have urged a large turnout in the polls to show support for the Islamic Republic and to defy its "enemies" - the United States and other Western states.
