To investigate the genetic variances in our successive selection strains for fast growth and white shell color of the Pacific oyster (Crassostrea gigas) during mass selection process, genetic diversity of three successive mass selection strains from G₁ to G₃, one based stock and two wild populations was analysed using multiplex assays with ten microsatellite markers. The study showed that the average allele number (N_a) ranged from 7.2 to 12.6 and the mean allelic richness (A_r) ranged from 6.8 to 11.0, the expected heterozygosity (H_e) and observed heterozygosity (H_o) ranged from 0.672 to 0.769 and 0.486 to 0.542, respectively. Compared to wild populations, the three successive mass selection strains showed significantly lower in the mean number of alleles and the mean allelic richness, but there was no significant difference in the average expected heterozygosity among the populations. There were 39 cases deviating significantly from Hardy-Weinberg equilibrium among the 60 population-locus cases (6 populations × 10 loci), and the inbreeding coefficient (F_is) ranged from 0.215 to 0.342. F_st values ranged from 0.005 to 0.076, showing low-moderate level genetic differentiation among the populations. The results suggest that the genetic variation has not been significantly affected by continuous mass selection and high genetic variability remains existing in the three successive selection strains, indicating that there is still potential for increased gains in future selective breeding programs.