We measured the sexual size dimorphism in eighteen morphological traits and female individual fecundity of Silurus asotus and Clarias fuscus collected from Lishui (Zhejiang, eastern China) in reproductive seasons. It had been found that their sex ratios were both 1:1. One-way ANOVA showed the adults of S. asotus were not sexually dimorphic in body length, while body length was significantly larger in adult males than in adult females of C. fuscus (P < 0.05). One-way ANCOVA with body length as a covariate showed that the females of S. asotus with special body length were significantly larger than the males in interorbital width and body depth (P < 0.05), while other examined morphological traits did not differ between the sexes. Furthermore, the females of C. fuscus with special body length were significantly larger than the males in body depth, pelvic fin precoxal length, and length between pelvic fin and anal fin (P < 0.05), whereas their anal fin coxal length, caudal peduncle depth and caudal fin length were significantly shorter in females than in males, and there were no significant differences between the sexes in any other of the examined morphological traits. Two-way ANOVA showed that body length of C. fuscus was significantly larger than one of S. asotus (P < 0.05), both sex and the interaction of sex and species had no effects on the body length. Two-way ANCOVA with body length as a covariate showed that head length, head width, snout length, interorbital width, caudal peduncle depth, caudal fin length, dorsal fin precoxal length, dorsal fin coxal length, length between pelvic fin and anal fin, body mass and carcass mass of C. fuscus were significantly larger than them of S. asotus, while head depth, body depth and anal fin length of C. fuscus were significantly shorter than ones of S. asotus (P < 0.05). Meanwhile, there were no significant differences in other morphological traits between them. Body depth, dorsal fin precoxal length, pelvic fin precoxal length, length between pelvic fin and anal fin were significantly larger among females than among males, while anal fin coxal length, caudal peduncle depth, caudal fin length and dorsal fin coxal length were significantly shorter among females than among males (P < 0.05). There were no significant differences in other morphological traits between the sexes. The interaction of species and sex had significant effects on body depth, anal fin coxal length, caudal peduncle depth, caudal fin length, dorsal fin coxal, length between pelvic fin and anal fin, but it had no effect on the other morphological traits. A principal component analysis resolved two components (with eigenvalues≥1) from fifteen size-free morphological variables, accounting for 68.4% of variation in the original data. The first component (51.2% variance explained) had high positive loading for size-free values of head width, interorbital width, caudal peduncle depth, caudal fin length, dorsal fin precoxal length, dorsal fin coxal length, dorsal fin coxal length, pelvic fin precoxal length, pelvic fin precoxal length, length between pelvic fin and anal fin, wherea the first component had high negative loading for size-free values of anal fin coxal length. The second component (17.2% variance explained) had high negative loading for size-free values of postorbital head length. C. fuscus had higher scores on the first and the second axes of a principal component analysis than did S. asotu, but both sex and the interaction of sex and species had no critical effect on the scores. The individual fecundity was significantly correlated with the body length and body mass in C. fuscus and S. asotus positively. One-way ANCOVA and Tukey's test showed that C. fuscus with special body length laid significantly more eggs than S. asotus. Sexual selection was the major evolutionary reasons for large males of C. fuscus, simultaneously had effects on tail traits correlated with locomotion. Fecundity selection affected the variations of morphological traits associated with abdominal cavity volume to a large extent. Environmental factors, food availability and nutritional status affected the evolution of sexual size dimorphism at the same time.