We analyzed the hydrodynamic properties of a gravity cage for flatfish culture during exposure to wave action. We constructed a numerical model to simulate the hydrodynamic behavior of the float collar, the base frame system, and the mooring system, the primary sections involved in withstanding waved-induced forces acting on the net cage structure. We conducted a series of experiments to validate the numerical model. The numerical model appeared to predict the experimental behavior well (mean relative error was <9%), suggesting our model can be used to accurately predict the primary hydrodynamic behavior of gravity cages in waves. After validating the numerical model, we evaluated the effect of the weight of the base frame and the height of the net pen on mooring line tension and cage motion. The mooring-line tension increased by 13% and 19% and the inclination decreased by 8.8% and 9.3% as the weight of base frame increased from 80 to 100 g and from 80 to 140 g, respectively. Thus, increasing the weight of the base frame in favor of decreasing the inclination results in an increase in the tension on the mooring lines of ~15%. The values for these parameters decreased by <10% when the height of net pen was increased from 20 to 30 cm. The motion of the base frame was reduced slightly due to an increase in the forces acting on the float collar and the base frame of the cage net. Based on our observations, we offer suggestions regarding the design and optimization