Abstract: On the basis stress analysis for both applied torsional and residual compressive stress on the arbitrary section of cylindrical helical compression spring,it is found out that the maximum direct stress is appeared in the inclined section under an angle of α=135° with the wire axis of spring. Therefore the fatigue cracks initiate almost at the inner surface of spring and propagate along the inclined section with α=135° inducing finally the fatigue fracture,However,the maximum direct stress can be effectively decreased by surface residual compressive stress σr,α in the direction of α=45° with the wire axis of spring introduced by shot peening,which may increase the fatigue resistance of spring.