Previous research on gametic incompatibility in marine invertebrates suggests that for highly dispersive marine invertebrate species, barriers to fertilization among the closely relate species taxa are often incomplete and sometimes asymmetric. The nature of these barriers can dramatically affect the patterns of gene flow and genetic differentiation between species, and thus speciation. Two cupped oyster species, Crassostrea hongkongensis and C. gigas are economically and ecologically important species native to the Northern and Southern coasts of China, respectively. C. hongkongensis is one of the most important oyster species currently cultured due to its high market value in Southern China. The species is distributed from Fujian to Guangxi Provinces, with populations centered in Guangdong Province. C. gigas is the most commonly species of oyster cultured, owing to its worldwide distribution, rapid growth, and dominant position in commercial oyster cultures. In Northern China, they are primarily farmed in Liaoning and Shandong Province. To evaluate the gamete compatibility between C. hongkongensis and C. gigas, the F₅₀ critical value was ascertained under the different factors, which included temperature (16, 20, 24, 28, 32℃), salinity (10, 15, 20, 15, 30), sperm concentration (10⁰, 10¹, 10², 10³, 10⁴ ind./μL), and individual variation (240 crosses), in July, 2010 in Dalian, Liaoning Province. Results clearly demonstrated that hybridization between C. hongkongensis and C. gigas was achievable in one direction due to asymmetric gamete compatibility. C. hongkongensis eggs could be fertilized by C. gigas sperm, but fertilization did not occur in the other direction. Generally, species-specific sperm-egg recognition was controlled by a key sperm gene, bindin, in oysters. Moreover, genic mutation of bindin may lead to one direction fertilization and cause pre-zygote isolation of sympatric species. The gamete compatibility of C. hongkongensis and C. gigas were chiefly affected by the temperature and salinity, respectively (P＜0.05). In contrast, the gamete compatibility of interspecific hybrids was mainly affected by the interaction effect (P＜0.05). The F₅₀ critical values of temperature, salinity, and sperm concentration were 20.90℃, 14.7, 2.40 ind./μL, respectively. The fertilization rates were less than 50% in 91 of 240 crosses, that is, the F₅₀ critical value for the proportion of individual variation was 37.92%. The average fertilization rates of GG (C. gigas♀×C. gigas♂), HH (C. hongkongensis♀×C. hongkongensis♂), and HG (C. hongkongensis♀×C. gigas♂) were 96.83%, 70.98% and 58.70%, respectively. These results suggest that there be appreciable variation among C. hongkongensis female with respect to receptivity to C. gigas sperm. Variation in receptivity may stem from an ancestral polymorphism in "receptivity factors" that has been retained in C. hongkongensis since C. hongkongensis and C. gigas. In conclusion, successful stimulation of post-mating isolation was highly possible through one-way fertilization due to the sexually asynchronous coevolution between the two oyster species. The gamete compatibility was affected by the environmental factors and intrinsic factors, and individual divergence in gamete interaction may be a crucial step in elucidating the fertilization barriers between the two species.