The antiferality effect of sibalusuah wood (Tristania sumatrana) towards mice was evaluated. The substance was administered through gastric lavage tube in 3 different dosis I 525, 1050 and 2100 mg/kg b.w. during seven days before the mating period was conducted. On day 12 after gestation all the pregnant mice were killed after anaesthez with ether and the fetuses were withdrawn by laparotomy. The number of fetuses and the resorption occurences were noted to make account for the expected effect. The results showed that resorptions of fetuses have occurred, in all level of doses and increase with higher dose. No resorption occurred in controls.Also there were non pregnant mice in every dose given but not for the control group, which indicates that Ms preparation has an antiferality effect by prolonging estrogen cycle.