The confined etchant layer technique (CELT) was applied to electrochemical micromachining on different types of GaAs (p-type, n-type, undoped). Cyclic voltammetry curves showed that the etchant bromine was generated on the mold and L-cystine was thus used as an efficient scavenger to react quickly with the etchant. Therefore, the etchant was confined very close to the surface of the mold and it etched the workpiece of GaAs when the distance between the mold and workpiece was less than the thickness of the confined etchant layer. An array of concave microstructures was fabricated on different types of GaAs by CELT using a mold with an array of convex hemispheres. Several factors including the concentration ratio between the etchant and the scavenger, types of GaAs, and anodic oxidation during the process of CELT were studied. Experimental results showed that the resolution of electrochemical micromachining increased when the thickness of the confined etchant layer decreased. During the microfabrication process, anodic dissolution affected the electrochemical micromachining of p-type GaAs much more than that of the other two types of GaAs. The oxide layer on p-GaAs had a strong influence on electrochemical micromachining. X-ray photoelectron spectroscopy (XPS) and polarization curves also proved the existence of the oxide layer on p-GaAs.