In this study, overflow behaviors through a partially open tainter gate mounted on a standard ogee spillway were investigated by using the FLOW-3D. The results indicated that the discharge coefficient is in the range of 0.685 to 0.723. A relation of gate-controlled discharge to free discharge was proposed and a reasonable correlation between the free and controlled discharge was obtained. Pressures on the spillway crest and the gate were also investigated. As the gate opening rate decreases with a fixed gate opening height and the gate opening height increases at a fixed gate opening rate, negative pressures on the spillway crest and the dimensionless maximum pressures on the gate increase.