Sexual dimorphism is very common in nature, and possibly evolved under the pressures of natural and sexual selection, however, the underlying mechanisms are still confusing. Many hypotheses were proposed to explain this phenomenon, such as explanations from dietary niche divergence, intra-sexual competition and mate choice, fecundity selection. To understand the evolution of sexual dimorphism in passerine birds, in present study, we measured the morphometrics (culmen length, bill width, bill depth, wing length, tail length, tarsus length, body mass) of 122 individuals (68 males and 54 females) and compared the difference of these morphological traits between sexes in a wild population of White-cheeked Starling Sturnus cineraceus, during early breeding season of the year 2019. Meanwhile, we also measured the morphology of internal organs of 28 dead birds which provided by the Forest Public Security Bureau of Qingyang City, and compared the difference between males and females. We used independent sample T-test to analyze the data, and calculated the dimorphism index and coefficient of variation for each sex and all variables to indicate the variability of each measurement. The results showed that in all body measurements except bill width, adult males were significantly larger than females (P < 0.05) (Table 1), exhibiting slightly male-biased sexual size dimorphism. By contrast, internal digestive organs showed no significant difference between sexes (all P values > 0.05) (Table 2). As a flock living bird, male and female starlings always forage together, and their food habits might be similar. The anatomical evidences of lacking sexual size dimorphism in internal organs suggest that food habit partition may have played a little role in the evolution of size divergence in this bird. Males compete potential nesting holes and guard mates in breeding season, the larger size might be advantageous in sexual selection. Our findings suggest that the larger body size of males is likely resulted from intra-sexual competition for nest sites and mates, however, more researches are needed to figure out the evolutionary mechanisms of sexual dimorphism in this bird.