The gonad maturation and spawning of blue crab, Portunus trituberculatus (Miers, 1876) using samples caught by trammer net in the western coast of Korea from January to December. The gonadosomatic index (GSI) showed the highest value of 15.0 in June and lowest value of 1.29 in August. Hepatosomatic index (HSI) showed the lowest in the spawning season from June to July. Thereafter, from August the value started to increase to 5.77 in December, showing an adverse trend against GSI. In the histological studies of the female reproductive organ, the gonad filled with primary oocytes from January to February and secondary oocytes in May. The female gonads mature from June, and mature and immature groups could be distinguished from December to February or March. In August, female developed another cycle of gonad development, showing new primary oocytes in the gonads again. Some of female crabs had receptaculum filled with already active sperm cells in it from March to May. In the time of August to September, almost all the females had the sperm sacs in the receptaculum. From the result, the coupling period estimated from August to September. Total RNA and DNA values of the crabs were highest in May (2.51, $0.57{mu}g/mg$). The DNA value remained unchanged except for the value in May ($0.09-0.13{mu}g/mg$). while RNA value was lowest in January ($1.08{mu}g/mg$). The RAN/DNA ratio was the lowest (6.23%) in May and highest (18.05%) in July. The value of correlations coefficient between body weight and fecundity of the crabs was higher that of between carapace width and fecundity.