This study investigates the flow characteristics of microinjection-molded lightguiding plates. This study also analyzes the position of melt fronts on lightguiding plates. Viscous heating, temperature and velocity distribution are utilized to analyze delay or advancement of the melt front experimentally and using three-dimensional (3D) numerical simulation. A slow injection speed reduces viscous heating. As viscous heating decreases, the temperature distribution decreases and plastic viscosity increases. A high plastic viscosity increases flow resistance and melt plastic velocity decreases. The advancing melt front of a lightguiding plate is delayed in this situation. The key of this research is that the plastic melt front can arrive the end position at the same time on filling stage of lightguiding plate for microinjection molding. In this research, the authors find increasing the injection speed can help the previous situation. Experimental results demonstrate that filling in the experiment is very close to that in 3D numerical simulation.