In this study, we measured the sexual dimorphism in body size and six other mophometrical variables along with the individual fecundity in females of ovoviviparous and invasive mosquitofish Gambusia affinis, collected in Lishui (Zhejiang, eastern China), during the reproductive season. The results showed that the number of adult females was more than that of the males during the breeding season, and the body length of adult females was significantly longer than that of adult males. Therefore, G. affinis is a species with highly female-biased sexual size dimorphism. One-way analysis of variance with body length as a covariate showed significantly higher (P<0.0001) head width, interorbital width, body width, and body mass for G. affinis females than males, while no significant sexual differences were identified based on the head length or caudal fin length. A principal component analysis resolved two components (with Eigenvalues ≥ 1) from six morphometrical variables, accounting for 65.1% of variation in the original data. The first component (45.4% variance explained) had high positive loading for size-free values of head width, interorbital width, body width, and body mass; and the second component (19.7% variance explained) had high negative loading for the size-free value of head length. Analysis of unary linear regression showed that the reproductive outputs (including brood size and mass) of G. affinis positively correlated with the female body length, body mass, head width, interorbital width, and body width. Brood size positively correlated with the average body length of offspring, but there was no correlation between the brood size and the average body mass. No trade-off was identified between the size and number of offspring. Sexual dimorphism in sex ratio, body size, and local characteristics of G. affinis was influenced by a variety of selection pressure parameters, including fecundity selection, sexual selection, niche differentiation, and food competition during the breeding season. Sexual dimorphism also contributed to population expansion and rapid invasion of the species. Our results have provided some basic data for further studies on the evolution mechanisms of fish sexual dimorphism.