Abstract: Portunus trituberculatus is an important economic species in China, and age identification is beneficial to its population resources assessment and management. We discussed the feasibility of gastric mill age identification by gastric mill of P. trituberculatus. The gastric mill was composed of five parts (propyloric ossicle, explyloric ossicle, urocardiac ossicle, zygocardiac ossicle and pterocardiac ossicle). According to the results of previous studies, we selected the explyloric ossicle and urocardiac ossicle for section observation. The results show that the gastric mill microstructure of P. trituberculatus consisted of four parts: wax layer, exocuticle, calcified layer and membranous layer. The growth lines that can be used for research mainly existed in endocuticle. We used seven growth equations to fit the relationship between biological data and the number of growth lines of gastric mill. Combining with AIC method, it is concluded that there are a linear function relation between carapace length and carapace width and a power function relation between carapace width and body mass; the relationship between body mass and the number of growth lines in the gastric mill is applicable to the Gompertz growth model, while that amang carapace length, carapace width and the number of growth lines in the gastric mill is more applicable to the v-B growth model. Thus, the growth lines of gastric mill can be used as the material for Crustacean age identification. However, due to the fact that the samples tend to be miniaturized, the time span needs to be extended for further verification.