Etch pits formed on (100)-oriented InAs surface by HCl-H2SO4-H2O solutions have been investigated by optical microscopy and atomic force microscopy (AFM) images. Depending on the composition of etchant, four types of etch pits are observed, which is attributed to different reaction rate of (111)A and (111)B planes by HCl and H2SO4. Excessive HCl results in long hexagonal shaped pits, whereas the addition of H2SO4 results in tetragonal pits. The direction of the composed pit sidewalls has been determined combining with the results of depth profiles on (110) and (11¯0) planes. The role of the HCl and H2SO4 in the over-all etching behavior of InAs and details about the geometrical relation between the sidewalls of the etch pits are discussed.