Temperature distribution in a fuel cell significantly affects the performance and efficiency of the fuel cell system. Particularly, in low temperature fuel cells, improvement of the system requires proper thermal management, which indicates the need for developing accurate thermal models. In this study, a 3D numerical thermal model is presented to analyze the heat transfer and predict the temperature distribution in aircooled proton exchange membrane fuel cells (PEMFC). In the modeled fuel cell stack, forced air flow supplies oxidant as well as cooling. Conservation equations of mass, momentum, and energy are solved in the oxidant channel, while energy equation is solved in the entire domain, including the gas diffusion layers and separator plates, which play a significant role in heat transfer. Parametric studies are performed to investigate the effects of various properties and operating conditions on the maximum cell temperature. The present results are further validated with experiment. This model provides a theoretical foundation for thermal analysis of air-cooled PEMFC stacks, where temperature non-uniformity is high and thermal management and stack cooling is a significant challenge.