Investigations were made on reproduction and population dynamics of Leptochela gracilis on the western coast of Korea, Yellow Sea, between May 2000 and October 2001. The egg volume was significantly larger at later egg stage than at early egg stage. Brood loss did not occur during the incubation period. Based on dry weight, the reproductive output (mass of incubating eggs/mass of female) averaged 0.18. The mature females first appeared in May, reached a peak in July and August, and then did not appear after September. The main breeding season was summer, although slightly different between the two years. A similar pattern could be found in monthly changes of gonadosomatic index (GSI), showing relatively higher GSI during the annual breeding season. A significant difference in ovarian dry weight between females with non-eyed eggs and eyed eggs indicates that ovarian maturation occurred during the embryonic development, suggesting that females were potentially consecutive breeders, capable of multiple spawning within a reproductive season. The size at which 50% of females were mature (CL50) was estimated as 7.63 mm carapace length. Females grew faster and reached a larger size at age than males (L∞=12.43 mm CL and K=0.90 y−1 for females, and L∞=12.22 mm CL and K=0.58 y−1 for males).