In this paper, photoresist reflow method was used to fabricate microlens array on the sapphire substrate which possesses high mechanical strength and transmittance in broad spectrum. High etch selectivity of sapphire over photoresist was obtained through adjusting ICP etching parameters. To test the fabrication process, a geometric model of square aperture microlens was built by finite element method. The validation of this model was done by comparing the surface profiles of three samples reflowed under different condition with the geometric model. In all three cases the simulation results were close to the experiment results. So the model was justified. On the other hand, the fabrication process was found to be repeatable because the surface profile of fabricated microlens was close to the theoretical surface profile of reflowed photoresist. The geometric model can be used to check the repeatability of photoresist reflow process and to predict the surface profile of microlens with irregular aperture.