In the design of mobile cranes, dynamic load factors are of great importance. These factors have been established as criteria empirically and conventionally in many countries buy have never been investigated theoretically. In addition, the recent variety and up-rating tendency of mobile cranes have led to the unsuitability of the factors. In this paper, we begin by setting up various simulation models of cranes, then proceed to the numerical analysis. Furthermore we try to establish formulas for dynamic load factors using multiple regression analysis. These approximate formulas are simple polynomials in which basic dimensions of crane are variables, and they are found satisfactory enough for any practical application.