At present, for aquatic animals, successive selection is mainly used in the breeding of fish and shrimp species. However, there are some relevant research studies on bivalves, and many attempts have been made and have obtained positive and encouraging results. Specifically, in oyster, all selection experiments on growth characteristics were successful in altering the selected trait in the direction of selection. For other bivalves, positive or successful responses to selection have also been observed in catarina scallop , Japanese scallop , hard clam P. maxima), and other species. The Kumamoto oyster is an important wild oyster resource in Southeast Asia, including China, Japan, and Korea. They not only occur naturally in China but also live in abundance over a wide geographical distribution, ranging from Jiangsu to Guangxi, including Hainan Island. Traditionally, this oyster was a wild fishery resource; farmers captured these oysters from the reef or stones in the inter-tidal zone. Although the artificial breeding of this oyster was successfully conducted, little relative information for genetic improvement was found. A selective breeding program for improving growth rate in Kumamoto oyster was initiated in China in 2012 after the breakthrough of large-scale artificial breeding. To compare differences in growth between two strains, mass selection for the first and second generation selection between two strains was performed from 2012 to 2014. As expected, a relatively higher response to selection and realized heritability was obtained for two populations after the first two generations. To investigate whether the response to selection after one generation of selection among different strains was sustained, we examine responses to selection, realized heritability, and genetic gain for growth traits of cultchless Kumamoto oyster taken from the third generation in southern China. The selection effect, strain origin, and their interaction were evaluated using multi-ridge and non-ridge strains by selecting the largest 10% of the oysters from the second generation parents for the selected groups, while an equal number of the parents were randomly chosen as the control groups before the removal of parents for truncation selection. The results show that the realized heritability of growth traits of multi-ridge strains was higher than those of non-ridge strains with a high genetic improvement level, while the non-ridge strains showed moderate heritability at a realistic level with a certain degree of genetic improvement potential. During grow-out for the adult stage, shell height and total weight of response to selection, and realized heritability and genetic gain were 0.70, 0.76; 0.40, 0.43; and 7.02%, 12.29% for multi-ridge strains; and these indices were 0.36, 0.33; 0.20, 0.19; and 3.74%, 5.72% for non-ridge strains, respectively. Growth traits were mainly affected by strain origins; a second factor was selection effects, and no interaction was found during the whole process. Hence, the selection response of the two strains continually decreased with increasing generations. This study provides an excellent base for new variety culture of Kumamoto oysters.