Injection molding has been used for mass production of polymer products with microstructures. Conventional Hele-Shaw 2.5D midplane simulation is unable to describe the flow pattern correctly. It tends to overpredict the effects of microstructures on global flow patterns. For the unidirectional flow, an x-z planar based on the general momentum equation is able to achieve better accuracy and to retrieve more detailed flow and heat transfer information around the microstructures. A hybrid numerical technique is developed, which can significantly reduce the nodes and computation time, and yet provide good flow simulation around the microstructures. The mold-melt heat transfer coefficient and injection speed are shown to be very important factors in determining the filling depth in microstructures. A decrease of the heat transfer coefficient and the occurrence of wall-slip are likely in rnicrochannels. (C) 2004 Society of Plastics Engineers.