In the present paper, size at sexual maturity was determined by relative growth (functional) as well as by examining the condition of gonads (physiological) in both sexes of Portunus pelagicus. During the study it was observed that the physiological and functional maturities occur almost at the same size which was from 68 to 75 mm short carapace width or 86 to 94 mm long carapace width. Male P. pelagicus matures earlier than the females. The size at which females attains full sexual maturity is 74-86 mm short carapace width (or 93 to 108 mm long carapace width). The size at 50% sexual maturity for male and female crabs was 63.57 mm and 83.55 short carapace width, respectively. The minimum number of eggs was 1, 72, 963 in a crab of 84 mm short carapace width whereas maximum number of eggs was found to be 11, 27,796 in a crab of 135 mm short carapace width. The average fecundity was 523773 ± 279204 (S.D) for a berried crab with a mean short carapace width of 98.53± 18.05 mm.