Abstract: An exact method for computation of envelopes of various internal forces of skeletal structures under moving loads is developed. The functions with two variables defining the relationship between the structural internal forces and the place of the moving loads are set up by introducing the conception of the influence surfaces of internal forces. The maximum and minimum values of these functions along the loading locations form the envelopes of internal forces. These functions are calculated by combining a minimum set of basis solutions. Therefore, the proposed method is not only exact but also very economical. Typical numerical examples are provided to show the validity, applicability, accuracy and efficiency of the proposed method.