Abstract: In this paper, systematic study was conducted to explore the causes of seed dormancy in the Chinese tallowtree (Sapium sebiferum (Linn.) Roxb.) Other than the early reports, coats of the tallowtree seeds were found to have good water permeability, which suggested that seed coats were not the main cause that inhibited the seed germination Meanwhile, germination test indicated that invitro embryo of tallowtree seeds didnt undergo dormancy However, extracting solutions of the seed coat and endosperm could suppress seed germination significantly, which implied the existence of endogenesis germination inhibitory substances in the corresponding seed parts Based on these findings, we proposed that inhibitory substances could be the main reasons that caused tallowtree seeds dormancy The tests of breaking tallowtree seed dormancy showed that: dormancy of tallow tree seeds could not be broken by both treatment with concentrated H2SO4 and the cold stratification method However, by testing with different gibberellic acid solutions, dormancy could be overcome by soaking the seeds in 500 or 1 000 mg/L GA3 followed by 100(GA3) days stratification