In this study, finite element simulation of joint strength of single lap joint of composite plates based on variations of joint thickness and overlap length is presented by using ANSYS software. In the simulation, plate thickness and overlap length of the joint structure are varied. Based on the variations of plate thickness and overlap length, it is found that the largest joint strength is obtained at the values of plate thickness and overlap length of 2 mm and 55 mm, respectively. Also, it appears that the applied loading type will affect to the simulation results.