Abstract: Nine half - sib families and twenty - seven full - sib families of the sea cucumber, Apostichopus japonicus,were obtained using artificial fertilization of two or three females by one male by an unbalanced nest design. The body lengths of 27 full - sib families in mid - auricular larvae (810 individuals) and the body lengths of 23 full -sib families in juveniles (640 individuals) were measured statistically and heritabilities were estimated through multiple -trait derivative -free restricted maximum likelihood (MTDFREML). There were significant differences in body length heritability estimates ranging from 0.29 ± 0.49 to 0.49 ± 0.55 for the two stages, indicating that the body length are under additive genetic control and are expected to respond to selection, potential genetic improvement for growth rate in sea cucumber.