An important problem for oxide confined vertical cavity surface emitting laser is current crowding at the edge of the aperture, which leads to extra losses and excitation of unwanted higher-order transverse modes. In this study, the results of self- consistently three dimensional electrical, optical and thermal simulations of 980 nm vertical cavity surface emitting laser are presented. Effect of oxide aperture size on current crowding of the device is investigated theoretically. Then the effects of the current crowding on the internal laser performance are discussed.