Laser micromachining has been utilised for fabricating channels in polymeric microfluidic devices. The present work focuses on a study of the influence of polymer molecular weight (Mw) on the depth and width of microchannels in polymethyl methacrylate (PMMA) that are machined using a CO2 laser. PMMA of four molecular weights, namely, 96.7 kDa, 120 kDa, 350 kDa and 996 kDa were used. The laser cut was performed at low power ratings of between 0.25 W to 2.5 W and the cutting speeds ranged from 7.0 to 64 mm/s. The depth and width of the machined microchannels varied from 18 μm to 660 μm and 110 μm to 450 μm respectively. The effect of the number of cut passes during laser micromachining on the morphology of the microchannel was considered by subjecting each microchannel up to 7 cut passes. It was demonstrated that microchannels with smooth surfaces could be obtained using CO2 laser machining and that the conditions for obtaining the smooth microchannels depend on the polymer molecular weight.