Objective. To investigate the independent and additive effects of resistance training (RT) and soy isoflavone (ISO) on body composition in postmenopausal women (PW). Method. This study used a placebo-controlled, double-blind (soy), randomized (ISO versus placebo) x (RT versus No RT) design. A total of 80 PW, aged 45-70 years, were randomly (71 completed 9-months intervention): RT + ISO (n = 15), No RT + ISO (n = 20), RT + placebo (n = 18), and No RT + placebo (n = 18). ISO received 100 mg a day of isoflavone; and to RT attended supervised resistance training sessions. At baseline and 9-months, fat and muscle mass were estimated by DXA. ANOVA and test t were used. Results. RT groups showed significantly increased muscle strength (35.2%) and muscle mass (1.4%). Exercising attenuated gains in fat trunk and % body fat (P < .05). Significant decreases in muscle mass (-1.8%) and increases in fat mass of the whole-body (1.6%) and trunk (9.7%) was found in no-RT groups (P < .05). In ISO groups, there were no differences in body composition and muscle strength. ISO and RT had no additive effects. Conclusion. In PW: RT improved muscle mass and strength and attenuated gain of fat mass; ISO did not alter body composition and muscle strength; there were no additive effects of RT and ISO.