Abstract: The fatigue strength condition of component parts under non-symmetrical circulation is analysed for the tranditional design of mechanism in order to infer an expression about the fatigue strength condition. In terms of material mechanics, the working safe coefficient of components under the same circle feature is found by two points on the same projecting line in the endurance limit figure. Because the combined influence coefficient of components has only an effect on the stain amplitude (the vertical coordinate),the endurance limits of materials and components can't be located on the same projecting line. In view of this,the tranditional fatigue strength condition under non-symmetrical circulation is incorrect. Through the analysis of the figure of the endurance limit of components, a correct expression about the working safe coefficient of components is obtained.And further, the error between the expressions of two strength conditions is analysed. The conclusion shows that the design according to the original fatigue strength condition would cause greater losses.