An ecological investigation was conducted in winter from November 2007 to February 2008, around Wangdol Reef in the East Sea to analyze the population structure and fecundity of Chinoecetes opilio. The sex ratio was 30.11:69.89% (female:male) in November, 62.03:37.97% in December, 37.59:62.41% in January and 14.69:85.31% in February. Regression equations indicated highly positive relationships between size and weight parameters (carapace length, width, total weight). During the sampling period ovigerous females were continuously collected. The percentage of ovigerous females was 96% in November, 90.57% in December 89.80% in January, and 88.46% in February. The average number of embryos was 64,800. The regression equation between carapace width and the number of embryos was y = 2.6805x - 0.9182 ($R^2$ = 0.7166). Embryo volume increased as embryo development proceeded. The mean size of an embryo was 0.72 mm. Embryo volume ranged from 0.42 to $0.84;m^3$ during embryo developmental stages 1 to 3.