The effects of body size, food density and its quality on assimilation efficiency of pacific oyster (Crassostrea gigas) were studied under artificial conditions in laboratory. The experimental results show that the filtration and ingestion rates of pacific oyster increased with its body size, which tallied with power function. The assimilation efficiency of pacific oyster was not significantly correlated with its body size, but correlated positively with the organic matter content of food and negatively with food density.