The Pacific oyster, Crassostrea gigas, naturally distributing along the coast of northwest Pacific, is one of the most important bivalve species due to its high economic value and fecundity. In China, we have initiated a selective breeding program on both shell color and growth rate of C. gigas since 2010. A black shell line was obtained through four-generation family selection. In this study, mass selection for growth improvement was conducted in the sixth generation and seventh generation of black shell lines. To assess the progress of potential genetic improvement, the progeny of two generations of black shell lines were selected to evaluate their shell heights via a 450-day farming experiment. As the results, after growing for 450 days, the sixth generation and seventh generation of selected lines were 9.03% and 11.42% larger than the control lines, respectively. During the grow-out stage, the genetic gain of two generations was 8.82% ± 0.18% and 11.54% ± 0.43%, respectively; and the corresponding realized heritability was 0.45 ± 0.04 and 0.41 ± 0.04, respectively. These results indicated that the mass selection for shell height achieved steady progress in the two generations of C. gigas.