Abstract: In this study, we use stable isotope method to analyze and compare the trophic ecology of two species of important commercial Sciaenidae fish, small yellow croacker (Larimichthys polyactis) and bighead croacker (Collichthys lucidus) in Lüsi fishing ground. The result showed that it is no significant difference in stable carbon isotope (p＞0.05) but significant in stable nitrogen isotope (p＜0.05) between the different lengths of L. polyactis and C. lucidus. The trophic level of L. polyactis was significantly higher than that of C. lucidus (p<0.05). The clustering analysis demonstrated that the body lengths of two fishes could each be separated into two groups. It could be the group of >140-190, >130-140, >120-130, >80-90 mm and the rest body length for L. polyactis. And the group of >140-190, >130-140 mm and the rest body length for C. lucidus. IsoSource model showed that the contributions of phytoplankton to L. polyactis and C. lucidus were 43.38% and 36.38%, respectively, and the contributions of sedimental organic matter (SOM) were 40.66% and 43.70%, respectively, which determined that phytoplankton and SOM were main potential carbon sources for two species of fish. We also found that the critical point of body length of diet shift of L. polyactis is 80 mm, smaller than that of C. lucidus (110 mm), reflecting that C. lucidus changed its dietary behavior later than the growth and development stage of L. polyactis.