Abstract: Four inbred families named AB, Ab, aB and ab were obtained using diallel cross between large(♀A, ♂B) and small(♀a, ♂b) sizes selected from a fast-growing family depending on standard of shell length. Growth and metamorphosis were analyzed at the stages of larval and juvenile. The results showed that growth sequence of inbred families was AB>Ab>aB>ab, and AB grew faster than ab with significant difference ( P<0.05), except day 9. Every group began to metamorphose at day 6, the metamorphosis rates of AB was 71.12%±1.53% which was higher than ab (41.6%±1.33%) (P<0.05). Distribution frequency of shell length of four experimental groups has changed by inbreeding within family. The size of AB obtained from up-selection verged on becoming large; however, ab obtained from down-selection verged on miniaturization. In conclusion, up-selection within families is the effective method for the genetics improvement of Manila clam.