This study investigates the effects of gate openings and different sill widths on the sluice gate’s energy dissipation and discharge coefficient (Cd). The physical model of the sills includes rectangular sills of different dimensions. The results show that the gate opening size is inversely related to the Cd for a gate without a sill. In addition, increasing the gate opening size for a given discharge decreases the relative energy dissipation, and increasing the Froude number increases the relative energy dissipation. The results also show that the Cd and relative energy dissipation decrease when the width of the sill is decreased, thus increasing the total area of the flux flowing through the sluice gate and vice versa. According to the experimental results, the relative energy dissipation and the Cd of the sluice gate are larger for all sill widths than without the sill. Finally, non-linear polynomial relationships are presented based on dimensionless parameters for predicting the relative energy dissipation and outflow coefficient.