This study supplemented the way of sex determination of fish based on the regression relationship of some morphological indicators of Mystus mysticetus. A total of 425 fish specimens were collected by trawl nets at Cai Rang - Can Tho and Long Phu - Soc Trang from January to July 2022. After collection, the fish were transferred to the laboratory to determine their total length (TL), standard length (SL), eye diameter (ED), eye distance (DE), body height (BD), head length (HL), and mouth width (MD) before anatomy for sex determination based on gonads. The regression analysis results of TL-SL, TL-ED, TL-DE, TL-BD, TL-HL, and TL-MD showed that SL, DE, and MD could be used to determine the sex of this fish species because SL and DE were dominant in growth in males, and MD was prevalent in females. In addition, SL could be used to determine the sampling time and HL could be to determine the distribution environment.