To understand possible reproductive interaction between Crassostrea ariakensis (Fujita, 1913) and C. sikamea (Amemiya, 1928), which coexist in estuaries of China and Japan, we conducted 2 × 2 factorial crosses between the two species. Asymmetry in fertilization success was observed, where C. sikamea eggs can be fertilized by C. ariakensis sperm, and the reciprocal cross resulted in no fertilization. Fertilization success in C. sikamea female × C. ariakensis male (SA) crosses was lower than that in the two intraspecific crosses and produced larvae that had similar growth rate as their maternal species during the first nine days because of maternal effects. After that, genome incompatibility casted negative effects on the growth and survival of the hybrid larvae. Most hybrid larvae died during metamorphosis, but a small number of spat survived. Genetic analysis revealed that the survived SA spat contained DNA from both species and were true hybrids. This study demonstrates that hybridization between C. ariakensis and C. sikamea is possible in one direction.