Alieh Eghdamdoust was among 70 people arrested during the rally in Tehran, but this was the first prison sentence to be implemented, leading activist Sussan Tahmasebi said.
Eghdamdoust was first sentenced to three years and four months in prison and 20 lashes. This was later reduced to three years in jail by an appeals court.
"She started serving her sentence [February 1]," said Tahmasebi, who herself is appealing against a partly suspended two-year jail sentence for the same protest, on June 12, 2006.
Of those arrested during the rally, 14 people were charged with criminal offenses, including spreading propaganda against the ruling system.
Police had said the rally was illegal as the protesters did not have a permit. Tahmasebi said peaceful rallies were allowed under Iran's constitution.