In order to make clear the effect of sizes on the feeding and metabolic physiology of Crassostrea hongkongensis, the variation of filtration rate (FR), ingestion rate (IR), absorption efficiency (AE), oxygen consumption rate (OR) and ammonia excretion rate (NR) under different sizes were studied at laboratory conditions by using a running water system. The results showed that: (1) Sizes had a very significant effect on filtration rate and ingestion rate of Crassostrea hongkongensis (P<0.01). The relationship between dry tissue weight and filtration rate and ingestion rate were as follows: FR=3.568 5W-0.590 1, (R2=0.956 4); IR=20.396W-0.539 2, (R2=0.967 9). Sizes had no significant impacts on absorption efficiency of Crassostrea hongkongensis (P>0.05). (2) Sizes had a very significant effect on oxygen consumption rate and ammonia excretion rate of Crassostrea hongkongensis (P<0.01). The relationship between dry tissue weight and oxygen consumption rate and ammonia excretion rate were as follows: OR=1.220 8W-0.444 8, (R2= 0.9481); NR=0.074 3W-0.469 1, (R2=0.922 4). The sizes had no significant impacts on the ratio of O ∶N (P>0.05). The values of O ∶N ratios were 18.09, which indicated that the Crassostrea hongkongensis in the experiment mainly utilized carbohydrate and lipid as their energy sources.