A 3×3 complete diallel cross comprising three families of the clam Meretrix meretrix(P1, P2 and P3) was used to determine the combining ability of parental families and heterosis of F1 under indoor and openair environments for growth traits. Analysis of variance for shell length and whole body weight indicated highly significant cross effects, environment effects and the interaction of cross by environment. General combining ability(GCA) and specific combing ability exhibited great variation among crosses and between two environments. Pooled over environments, P2 was the top combiner among the three parental families for both traits studied. The cross of P1 and P3 had the highest SCA. Additionally, significant reciprocal effects were observed. For individual environment, about half of the crossbred combinations showed favorable Mid-parent heterosis(MPH)(>1%) for the shell length and whole body weight. Our data has shown that non-additive genetic and reciprocal effects constituted the major sources of genetic variation for both shell length and whole body weight, which indicates that crossbreeding among selective families could further explore the heterotic effects. A 3 × 3 complete diallel cross comprising three families of the clam Meretrix meretrix (P1, P2 and P3) was used to determine the combining ability of parental families and heterosis of F1 under indoor and openair environments for growth traits. Analysis of variance for shell length and whole body weight indicated highly significant cross effects, environment effects and the interaction of cross by environment. General Giving ability (GCA) and specific combing ability show great variation among crosses and between two environments. Pooled over environments, P2 was the top combiner Among the three parental families for both traits studied. The cross of P1 and P3 had the highest SCA. For individual environment, about half of the crossbred combinations showed favorable Mid-parent heterosis (MPH) (> 1%) for the shell length and whole body weight. Our data has shown that non-additive genetic and reciprocal effects composed of major sources of genetic variation for both shell length and whole body weight, which indicates that crossbreeding among selective families could further explore the heterotic effects.