In order to investigate heterosis of two lines of Manila clam (Ruditapes philippinarum) with different shell shapes ,the diallel cross of wideshell (W) and plateshell(P) was conducted in June,2007. The experiments consisted of two hybrid groups ［PW（P♀×W♂）,WP(W♀×P♂)］and two selffertilized groups ［WW(W♀×W♂), PP(P♀×P♂)］. The results show that the shell width/shell length(W/L) and numbers of radiocostae of hybrid offspring are between those of offspring of two selffertilized groups. At planktonic stage, instead of the larval growth heterosis, the larval survival heterosis and maternal effect of larval size are observed, the survival heterosis of offspring of PW and WP are (3.43±0.54) and (4.21±0.55) ,respectively. At nursing stage indoor, both growth and survival heterosis are found, and the maternal effects decrease ,the growth heterosis of offspring of PW and WP are (10.76±2.25) and (8.28±1.88), the survival heterosis are (5.52±0.62) and (11.10±2.41). At grown out stage outdoor, the growth and survival heterosis of offspring is significant, the maternal effects disappeared. The growth heterosis of offspring of PW and WP are (16.22±0.23) and (14.80±1.50), and the weight heterosis are (50.29±1.13) and (35.27±2.43), respectively; the survival heterosis are (12.30±1.37) and (17.45±0.75). The above results indicate crossing of Manila clam with different shell shapes has heterosis which increase with development.The study will lay theoretic basis for utilization of heterosis of Manila clam with different shell shapes.