Placenta produces progesterone and estradiol for maintaining pregnancy. Two critical enzymes are responsible for their production: 3β-hydroxysteroid dehydrogenase 1 (HSD3B1) that catalyzes the formation of progesterone from pregnenolone and aromatase that catalyzes the production of estradiol from testosterone. Fungicide ziram may disrupt the placental steroid production. In the present study, we investigated the effects of ziram on steroid formation in human placental cell line JEG-3 cells and on HSD3B1 and aromatase in the human placenta. Ziram did not inhibit progesterone production in JEG-3 cells and HSD3B1 activity at 100 μM. Ziram was a potent aromatase inhibitor with the half maximal inhibitory concentration (IC50) value of 333.8 nM. When testosterone was used to determine the mode of action, ziram was found to be a competitive inhibitor. Docking study showed that ziram binds to the testosterone binding pocket of the aromatase. In conclusion, ziram is a potent inhibitor of human aromatase.