Abstract: Based on supersonic linearized theory, the flow field in the region near the side-edge of the ramp is calculated, and then a formula of mass flow ratio of side-spillage is derived by the use of the technique of mass flow integral. Using Carafoli's method on the basis of Busema-nn's conical flow theory, the method and the computing curves for predicting the effects of sweepback side-plate are given.Comparison between the results of theoretical calculation and available experimental data shows the agreements are satisfactory.