The combustion characteristic in classical hybrid rocket motors has been studied. The heat transfer process in the gas and the solid fuel has been considered. Coupling computation is presented by the regression rate equations based on the heat transfer theory and the Arrhenius law. The regression rate at various oxidizer mass flow rates, axis distances, ambient operating temperatures and times is obtained. Fuel regression rate mainly infects with oxidizer mass flow rate and axial distance. Regression rate increases as oxidizer mass flow rate increases and decreases as axial distance increases. The temperature sensitivity of solid fuel in a hybrid rocket motor is low and we need not assign a weight margin for variations in ambient operating temperatures. Adiabatic combustion temperature at various oxidizer mass flow rates and grain length is obtained by thermodynamic calculation and has a maximum as oxidizer mass flow rate increases. The results are in agreement with the some references and useful for further study.