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CO2 laser ablation has been a popular technique for microfluidic chip fabrication but it always suffers some problems such as bulges, clogging, re-solidification and large heat-affected zone around the rim of channels and holes. In this Letter, a simple effective method for diminishing the defects has been proposed using a metal-film protection for CO2 laser ablation. Polymethylmethancrylate (PMMA) was used as a substrate to demonstrate the fabricated microfluidic channels without clogging as well as bulges reduced. The feature size of the microchannel can also distinctly be decreased from 268 m in air to around 58 m with metal-film-protection method, which is smaller than the laser-beam spot size. The bulge height was reduced to less than 0.2 m and clogging effect in the channel junction was not occurred. ANSYS simulation was also used to analyse the temperature distribution on PMMA substrate during laser ablation in air and with metal-film protection. The method presented here is suitable for microfluidic device production while overcoming the bad surface quality and low-resolution limit of CO2 laser ablation in air.