In this paper, the thermal analysis of 980 nm vertical cavity surface emitting lasers with different oxide aperture diameters is theoretically performed by using simulation software PICS3D which self-consistently combines three-dimensional simulation  of carrier transport, self-heating and optical wave-guiding. Temperature profiles and heat power density were obtained by using a three-dimensional cylindrical heat generation and dissipation model. Simulation results show that Joule heating is the main heat source in the device. The thermal resistance increases significantly with a decrease in oxide aperture size.