In order to understand the co-culture impact of the sea squirt Halocynthia roretzi on the Pacific oyster Crassostrea gigas, we examined the filtration rate of the sea squirt in relation of water temperature ($5-25^{circ}C$) and body weight. The filtration rate increased in relation to body weight and water temperature, indicating a clear positive correlation with the two variables. Due to the clear positive correlation between filtration rates at each temperature groups for acceptable collinearity, each constants at regression equation was further analyzed for a unified model of filtration rate. Therefore the filtration rate of H. roretzi was estimated as: Fr (L/h/animal)=$(0.1956+0.0182T){ imes}DW^{0.7978EXP(-0.0273T)}$ for water temperature in the range of $5-25^{circ}C$. The estimated filtration rates of H. roretzi were higher than those of C. gigas in the winter season (water temperatures below $16^{circ}C$) in condition of equal body weight. Our results indicate that H. roretzi may have an advantage over C. gigas in food competition during the winter season when the two species are co-cultured.