The growth, mortality, and reproduction of Portunus sanguinolentus were studied using size-frequency data obtained from crabs collected in pots in the waters off northern Taiwan from October 2000 to March 2001, and October 2001 to January 2002. The Bhattacharya's method and seasonal von Bertalanffy growth curve were used to estimate growth parameters. The growth curve for males was L_t = 204.75×{1 −e^{–[0.87t + 0.4(0.87/2)sin2(t)]}} and the curve for females was L_t = 194.25×{1 −e^{–[0.97t + 0.4(0.97/2)sin2(t)]}}. A size-converted catch curve was used to estimate the instantaneous total mortality rate (Z), and Pauly's empirical equation was used to estimate the instantaneous natural mortality rate (M). For males, Z = 3.16/year and M = 1.65/year. For females, Z = 3.37/year and M = 1.8/year. The instantaneous fishing mortality rate (F) was 1.51/year and 1.57/year, and the exploitation rate (E) was 0.48 and 0.47 for males and females, respectively. The exponential relationships were presented for relationships of fecundity in number and weight of egg mass in terms of carapace width and body weight. Those relationships were statistically significant (P < 0.01), indicating that the fecundity increased with the size from 4.05 × 10⁵ to 2.44 × 10⁶ eggs.