It is shown that only at certain pressure decay rates during depressurization and pressure rise rates during compression will the gas-phase will be in the unsteady state. Because of the wide discrepancy between the predicted values and the experimental values of the transient burning rate of composite propellant when the gas-phase is in the unsteady state, a model was proposed which can be applied to both compression and depressurization that takes into account the unsteadiness of the gas-phase. In this model, the energy liberation at the surface of the propellant is maintained constant. The predicted values of the transient burning rate agree very well with the available experimental results. © 1977.