Increasing water elevation in dam reservoirs increases power plant working time and hydroelectric energy. If gated spillway is implemented, water elevation of reservoir can be increased without changing the dam height. Application of gate for spillways however increases discharge per unit width and the potential of cavitation attack. In the present work, using Flow3D software first the accuracy of the model was verified with experimental data of a stepped spillway. In the next step the spillway was gated keeping the flow discharge constant. To do this, the elevation of the ogee was increased and the chute width was narrowed. The main purpose of this study was to evaluate the pressure and cavitation potential in the gated step spillway. Studying flow condition in various gate opening showed that lower gate openings cause high negative pressures at the first step downstream of the gate. In the lowest opening of 10% -9.2 m pressure was observed at the first step tip. In addition, according to the results, tail of the gate pier caused flow separation and negative pressure was observed at the step at its downstream. The present work showed that numerical models are powerful instruments for optimization and design of hydraulic structures