There are many existing RC bridge slabs reinforced with plain bars, and they are suffering from a huge number of load repetitions due to heavy traffic. Therefore, a suitable strengthening technique is required to extend their fatigue life such as externally FRP sheets. To take full advantage of this strengthening technique, it is beneficial to use a numerical method to predict the improvement on fatigue life and behaviors. In this study, two full scale RC slabs reinforced with plain bars under moving load are analyzed using a proposed numerical method based on the bridging stress degradation concept to simulate their fatigue behaviors. One of them was strengthened with externally bonded FRP sheets in longitudinal and transverse directions on slab bottom surface. The propagation of cracked elements, center displacement evolution and FRP strain are presented in this study. These numerical results are compared with the experimental results, and this comparison provides a good agreement.