Abstract: Objective To study the effect of soy isoflavone on levels of B-cell lym phom a 2(Bcl-2) protein and Bcl-2 associated X prote in(Bax) in small female rat's ovary.Methods Fifty two-month-old female rats were randomly divided into five groups: control(baseline feed), low dose(soy isoflavones of 100 mg/kg·bw), mode rate dose (soy isoflavones of 200 mg/kg·bw), high dose(soy isoflavones of 300 mg/kg·bw), and estrogen group(diethylstilbestrol of 0.5 mg/kg).The treatment lasted for seven weeks.The levels of Bcl-2 and Bax were detected with ELISA.Results The gray values of Bcl 2 in control, low dose, moderate dose, high dose, and estrogen group were 1.41±0.19, 1.43±0.20, 2.07±0.58, 2.42±0.32, and 2.95±0.78 without significant difference among the groups(P>0.05 for all) and those of the Bax were 33.64±5.83, 30.02±9.44, 17.33±4.21, 9.67±1.95, and 10.69±4.95, respectively.The soy isoflavone could remarkably reduce the activity of Bax in high dose group compared with that of the control group(P<0.05).Conclusion Isoflavone has aging-resistant effect on small female rat through antagonized pro-apoptotic bax protein and has no effect on Bcl-2.