Predation is one of the most important biotic factors regulating oyster population establishment and oyster reef development. This study examined the predating preference and efficiency of a stone crab (Charybdis japonica) and two oyster drillers (Rapana venosa,Thais luteostoma) on four size class (W1:10-20mm; W2:20-30mm; W3:30-40mm; W4:>40mm) of Crassostrea ariakensis and C. sikamea through lab experiments. The 2-way ANOVA indicated that Charybdis japonica showed no discriminatory predation on C. ariakensis and C. sikamea (P>0.05), while the oyster size significantly influenced the predating efficiency of the stone crab. The stone crab had significantly greater predating efficiency on W1 group of C. ariakensis than W2 and W4 group of the conspecific, and there were intermediate predation efficiency on the W3 group (P>0.05). The W1 group of C. sikamea was predated at significantly greater ratios than W2 and W3 groups of the conspecific, while the W4 group of C. sikamea had similar predating efficiency with the other 3 groups (P>0.05). The driller Rapana venosa had consistently low predating efficiency on the two oyster species (P=0.590) and the four size class of oysters (P=0.357). The driller Thais luteostoma had consistently predating efficiency between C. ariakensis and C. sikamea (P=0.917), but the oyster size significantly affected the predating efficiency of Thais luteostoma (P=0.035). Duncan' multiple comparison indicated that Thais luteostoma had similar predating efficiency among the four size class of C. ariakensis (P>0.05), but the drillers had greater predating efficiency on W1 group of C. sikamea than the other 3 groups of the conspecific (W2, W3 and W4) (P<0.05). The significantly positive correlations were found between the shell height and the shell thickness for each of C. ariakensis and C. sikamea (P<0.001). It was concluded that the three invertebrates showed no discriminatory predation on C. ariakensis and C. sikamea, while the predating efficiency on different size class of oysters had interspecies variations.