Inbreeding depression may be an avoidable phenomenon for abalone culture. However, only a few studies were carried out on inbreeding depression. In the present study, to demonstrate inbreeding depression in growth trait of Pacific abalone, H. discus hannai, inbreeding and outbreeding families were produced in 2010. Inbred and outbred families from each experiment were reared in same tank until 10 month for the same breed environment. The individual of inbred and outbred were distinguished by paternity test using microsatellite DNA. The shell length between inbred and outbred families was compared. At the results, significantly higher shell length was observed in the outbred families at 10 mon (P < 0.05). These results indicate that inbreeding depression is obviously observed in growth traits in the first generation of full-sib family of the H. discus hannai.