The dynamical model of combustion in liquid propellant rocket motors is an unstable time-delay system. When it is stabilized by a linear state feedback controller, the robustness of the closed-loop system is not satisfactory. In this paper a variable structure control approach to stabilize the combustion is presented. First, the concept of switching functional is introduced, and hence a systematic design method is established for the variable structure control of general time-delay systems. Secondly, the proposed theory is applied to the stabilization of combustion in liquid monopropellant rocket motors. Simulation results show that the robustness of the closed-loop system against variations in time lag delta and pressure exponent gamma is considerably improved. Specifically, the controller designed according to the nominal value delta = 1.0 can still stabilize the system with delta = 0, i.e. the system without time delay. This conclusion provides a powerful basis for the reliable design of rocket motor control systems.