In recent years, microinjection molding has been widely used for fabricating polymer components due to its cost effectiveness and mass-production capability. In this work, the fabrication process of a polymer micromixer was presented. The micromixer was designed in such a way that the fabrication process could benefit from the process capabilities of ultraprecision micromachining and microinjection molding. An amorphous polymer material polymethylmethacrylate was used to make the micromixers. Moreover, in order to investigate the effects of processing parameters on replication quality of the micromixer, four important factors in microinjection molding, namely the melt temperature, injection velocity, packing pressure and packing time were selected as variables. The experimental results showed that the melt temperature was the most important factor influencing the replication, followed by the injection velocity. However, the packing pressure and packing time had no obvious influence on the replication of the micromixer.