Objective: To observe the expression of peroxisome proliferator-activated receptor γ (PPARγ) and cytochrome P450 aromatase (P450arom) in granulosa cells and explored the possible pathogenesis of hyperandrogenemia in patients with polycystic ovary syndrome (PCOS). Methods: The study included control group and PCOS group. Control group contained 30 infertile patients without PCOS and PCOS group contained 30 patients with PCOS. Levels of hormones in serum and follicular fluid were determined by electrochemiluminescence method. Levels of PPARγ and P450 arom mRNA in granulosa cells were detected by qRT-PCR. Levels of PPARγ, P450 arom, Smad2 and p-Smad2 protein in granulosa cells were evaluated by Western blot. The correlations of PPARγ and P450 arom with some factors were analyzed. Results: P450arom expression was significantly lower, but PPARγ expression was significantly higher in PCOS group than in control group (P<0.05). PPARγ expression was negatively correlated with P450arom expression (r=-0.547, P=0.001) in PCOS group. p-Smad2 expression was positively correlated with P450arom expression (r=0.530, P=0.003), but negatively correlated with PPARγ expression (r=-0.406, P=0.002) in PCOS group. P450arom expression was positively correlated with estradiol (E2) level (r=0.493, P<0.05), but negatively correlated with testosterone (T) level (r=-0.511, P<0.05) in follicular fluid; and was not correlated with some in-vitro fertilization-embryo transfer (IVF-ET) parameters in patients with PCOS (P>0.05). PPARγ was negatively correlated with E2 level (r=-0.270, P<0.05), but positively correlated with T level (r=0.301, P<0.05) in follicular fluid; and was not correlated with some IVF-ET parameters in patients with PCOS (P>0.05). Conclusion: In granulosa cells of patients with PCOS, PPARγ expression is up-regulated, but P450arom expression is down-regulated. Both PPARγ and P450arom expression are not related to some IVF-ET parameter. Smad2 protein may be involved in the regulatory effect of PPARγ on P450arom.