In this study, effects of sperm concentration and gamete age on fertilization success of Haliotis discus hannai (D) × H. gigantea (G) were investigated. Results showed that the fertilization rates of heterologous crosses H. discus hannai♀ × H. gigantea♂ (DG) and H. gigantea♀ × H. discus hannai♂ (GD) were consistently lower than those of homospecific groups H. discus hannai♀ × H. discus hannai♂ (DD) and H. gigantea♀ × H. gigantea♂ (GG). In sperm concentration experiments, the sperm concentrations that yielded maximum fertilization rates with the least abnormality in subsequent development were 4.66 × 10⁷ sperm/mL for the DG cross and 2.6 × 10⁷ sperm/mL for the reciprocal cross GD. In gamete age experiments, the optimal fertilization rates were achieved in heterologous crosses when freshly spawned ova were fertilized with sperm that were released within 0.5 h. Furthermore, in heterospecific crosses, when ova were fertilized 10 min after being spawned, fertilization rates declined significantly with increasing ova age. It is suggested to use fresh gametes and higher sperm concentrations for hybridization between H. discus hannai and H. gigantea.