Betta fish (Betta splendens) is one of the ornamental fish that is in demand by the public because it has aesthetic value and high economic value. The attractive body color of male betta fish with long fins causes this fish to be more attractive to the public compared to female betta fish. Male betta fish have the prospect of being monosexually cultured which can be done by masculinization method. The purpose of the study was to determine the effect of pasak bumi extract on the percentage of male sex ratio, hatchability and survival rate of larvae. The masculinization method is an engineering technique of administering steroid hormones that can increase the male sex ratio with using natural materials that are environmentally friendly such as pasak bumi (Eurycoma longifolia, Jack). Pasak Bumi contains phytosterol compounds so it can be used as a natural ingredient in masculinization applications. The research was conducted at the Laboratory of Aquaculture Study Program, Faculty of Agriculture, Asahan University using Completely Randomized Design 5 (five) treatments and 5 (five) replications with dose pasak bumi extract is 0 mg/L; 1 mg/L; 2 mg/L; 3 mg/L and 4 mg/L. The treatment was given by soaking betta fish embryos in the Morula phase, which is about 2 (two) hours after fertilization. The results of the study show that concentration pasak bumi 3 mg/L resulted in the highest percentage of males from all treatments, which was 76.50%. The percentage of hatchability for all treatments is 100% However, the highest survival rate was found indose 4 mg/L by 65%. The research results also show found fish intersex in the treatment of 2 mg/L of 0,40% and 4 mg/L by 0,25%. Water quality during the study in the form of temperature ranged from 24.5-26.7°C; pH 5.8 – 7.1 and DO 3.5 – 4.7 mg/l.