With the introduction of hobby laser engravers/cutters, the use of CO₂ laser micromachining on poly (methyl methacrylate) (PMMA) has the potential for flexible, low cost rapid prototyping of microfluidic devices. Unfortunately, the feature size created by most entry-level CO₂ laser micromachining systems is too large to become a functional tool in microfluidics. In this paper, we report a novel method to reduce the feature size of microchannels and the bulges formed at the rim of channel during CO₂ laser micromachining by passing the laser beam through a stainless steel pinhole. Without the pinhole, the channel width was typically 300 um wide. However, when 50 um and 35 um diameter pinhole was used, channel widths of respectively 60 um and 25 um could be obtained respectively. The height of the bulge deposited directly next to the channel was reduced to less than 0.8 um at the presence of pinhole during ablation. Separations of fluorescent dyes on devices ablated with and without the pinhole were compared by the theoretical plate counts. On devices fabricated with the presence of pinhole, the number of theoretical plates/m was 2.2 fold higher compared to devices fabricated without the pinhole.A mass-produced commercial hobby laser (retailing at ~$2500), when equipped with a $500 pinhole, represents a rapid and low-cost approach to the rapid fabrication of rigid plastic microchips including the narrow microchannels required for capillary electrophoresis.