Polymer microlens manufacturing using thermal reflow was simulated and optimized by a numerical approach. Microlenses are used in various industrial fields, such as optical, semiconductor, and observation experiment equipment. Therefore, polymer microlens fabrication using an economical thermal reflow process is important for mass production and cost reduction. The feasibility of a thermal reflow process for microlens fabrication was analyzed in this paper by numerical methods. First, we refer to the previous studies and papers for the theoretical shape of the microlens. Second, for numerical simulation of the process above Tg (Glass Transition Temperature), we studied the multiphase flow simulation using a VOF method and adopted a Cross-WLF model to consider the rheological characteristics of PMMA. Finally, several parametric studies were carried out to compare the simulation profile and the theoretical lens shape in order to optimize the thermal reflow process. The numerical approach presented in this paper would enable a more efficient analysis and provide better understanding of reflow behavior to obtain the optimal process.