Abstract: The sex differentiation of fish is extremely susceptible to environmental stresses (such as high temperature, high density, etc.), and generally causes masculinization. In the critical period of sex differentiation, environmental stresses invariably lead to elevated serum cortisol in the process of masculinization. Cortisol treatments caused female-to-male sex reversal, while antagonist of cortisol synthetase rescued the high temperature-induced masculinization. Estrogen 17β-estradiol (E2) rescued cortisol- and high temperature-induced masculinization. These findings suggest that cortisol is the vital mediator in stress-induced masculinization. Few studies have been explored physiological mechanisms of stress-induced masculinization. The present study investigated the effects of high temperature and cortisol administration on the histological process in the masculinization of XX genotype using yellow catfish Tachysurus fulvidraco with confirmed sex chromosome XX-XY and sex-linked marker as the research object. Combining genotypic sex identification via applying sex-linked markers and phenotypic sex identification through histology, we found that 24 days (12—35 days post-hatching) treatment of high temperature or cortisol induced partial masculinization of XX individuals. During this process, germ cell development of XX gonad was repressed, and therefore it developed into testis-like structure. In addition, we observed seminiferous lobules and typical male genitalia in XX pseudo-male, strongly suggesting that these pseudo-males may have reproductive capacity. We also found that testis size and body weight of XX pseudo-male were significantly higher than those of the XY males and XX females. These results provide important information for understanding the mechanisms of environmental stress factors-induced masculinization. We also found that some XX individuals were not sensitive to high temperature (or cortisol) treatments, and meanwhile, high temperature accelerated ovarian development. The genetic difference between the two different temperature-sensitive XX individuals deserve further investigation.