The growth condition dependence of the microchannel epitaxy (MCE) width-to-thickness ratio ( W/T ratio) was studied systematically. The result shows that by optimizing the line seed separation and growth temperature, the GaAs layer with a high W/T ratio has been grown on GaAs-coated Si substrate, respectively, at 500 μm and 530°C. In this work, GaAs MCE layers with a high W/T ratio of 17.5 were obtained. Furthermore, by employing the optimized growth condition, GaAs layers with a wide dislocation-free region on a Si substrate were obtained after the growth of 7 h. Molten KOH etching showed that the width of the dislocation-free region is about 43 μm for the sample grown for 3 h. The width is enough to fabricate devices such as vertical cavity surface emitting lasers (VCSELs).