Assimilation efficiency (AE) of triangle sail mussel Hyriopsis cumingii which fed on eight species of microalgae was determined during the 24 hours trial, at 21 ℃ of water temperature. Oxygen consumption and ammonia N excretion rates of Hyriopsis cumingii were also studied in laboratory. Results showed that AE of Hyriopsis cumingii on Bacillariophyta (0.660±0.003) was higher than that on other microalgae species at the same level of algal density. The highest oxygen rates of consumption and the highest ammonia N excretion rates were found with the group fed on the Cyanophyta, which were at 1.346±0.024 μg/(g·h) and 25.78±0.237 μg/(g·h), respectively. while the lowest ammonia N excretion rate and the lowest oxygen rate were in Chlorophyta group (0.795±0.015 μg/(g·h)) and the Bacillariophyta group (13.307±0.127 μg/(g·h)) .Furthermore, the O∶N ratio revealed that protein was consumed by Hyriopsis cumingii when they fed on Bacillariophyta, while more fat and protein were consumed when fed on Cyanophyta or Chlorophyta.