This study aims to investigate laser welding process for the bonding of micro-fluidic devices. PMMA was selected for the investigation. The devices consist of an opaque substrate with micro-channels and a transparent cover. The welding process was optimized according to laser power, welding speed and clamping pressure. The cross-sectional analysis, flow and pressure tests, as well as the lap-shear test were conducted on the samples welded with the optimized process parameters. The results show that the laser welding can meet the requirements for bonding the plastic micro-fluidic devices.