Several strains of pearl oysters (Pinctada fucata) are distributed in the coastal areas of South China. To explore strategies for genetic improvement, additive genetic, heterotic, and strain reciprocal effects on growth traits were estimated by performing a full 3 × 3 diallel cross of three pearl oyster strains that are local to Beihai, Guangxi Province; Xuwen, Guangdong Province; and Sanya, Hainan Province. The simultaneous production of progeny for all crosses was achieved using mass spawning in Lingshui, Hainan Province. For body weight, the Beihai strain showed the highest additive genetic effect (+ 5.7%) and the Sanya strain showed the lowest (− 4.3%), with similar additive genetic effect patterns for shell length, shell height, and shell width. The averaged heterotic outcomes were significant (P < 0.01) for all growth traits, varying from + 6.1% for shell length to + 19.1% for body weight. Reciprocal cross effects were determined for growth traits, with a relatively high correlation for all traits between strain additive performance and total performance (r² = 0.72 to 0.85). These findings reveal the potential to exploit strain additive variation to improve P. fucata culture stock using direct selection methods for increased productivity of pearl oyster cultivation in China.