In this paper, a semi-analytical thermal model of friction stir welding processes is developed in which the heat-generating regions are divided into many elements as point heat sources. The heat generation in each element involves the friction and plastic deformation, and the temperature rise caused by each element is calculated by solving the heat conduction equation of a moving heat source in a solid body. The heat loss through the top and bottom surfaces are considered in the model as heat sinks. The asymmetric distribution of the temperature is calculated through the whole process and over the whole volume of the workpiece by integrating the effects of all heat sources and sinks. The temperature-dependent material properties are updated by a numerical routine. The comparison between the calculated results and the experimental data clearly approved the validity of the proposed method for some aluminum and steel alloys.