Sexual size dimorphism is common in animals. Understanding the differences in sexual dimorphism of body size between closely related species could provide evidence for insight into the underlying mechanisms driving sexual dimorphism of body size. Previous studies on Great Tit (Parus major) and many other birds have reported obvious sexual size dimorphism in morphometric traits such as bill length, tarsus length and wing length, and that the sexual size dimorphism of bill length of Great Tit may vary by seasons. The Cinereous Tit (P. cinereus) was once regarded as a subspecies of the Great Tit and it is similar to the Great Tit in various aspects of morphology and behaviors. Aiming to provide supplementary information about the interspecific difference in sexual size dimorphism between closely related species, we explored sexual and seasonal differences of Cinereous Tit in six morphometric traits including bill length, bill-head length, tarsus length, wing length, tail length and body length. From 2018 to 2020, a total of 226 individuals (96 females and 130 males) were captured in the Dongzhai National Nature Reserve of Henan Province, China. The relationships of each of the above morphometric traits with sex and season were analyzed using linear mixed models, in which each morphometric trait was treated as the dependent variable, and sex, season and their interaction as explanatory variables. Moreover, as there were three persons who attended the measurements of the birds, person ID was treated as a random factor to account for inter-person differences in measurements. The results showed that there were different degrees of sexual size dimorphism among the six morphological traits (Table 1), and the males were significantly larger than the females in all traits except bill length which showed no significant differences between the sexes. Moreover, seasonal variation was found for tarsus length and tail length: the tarsus length in autumn was significantly shorter than that in winter and breeding season, while the tail length in breeding season was significantly longer than that in autumn and winter. Despite of the seasonal differences in bill length and tail length, the sexual differences in the six morphological measurements did not significantly change with seasons. These results suggest that the patterns of sexual size dimorphism and seasonal differences of morphological traits of the Cinereous Tit are not same as those of the Great Tit. Future studies are warranted to investigate the reasons for the existence of sexual size dimorphism and seasonal variation of morphological traits in Cinereous Tit as well as the reasons for the inter-specific differences in sexual size dimorphism and seasonal patterns.