A study was made on the micromachining of GaAs in a H3PO4:H2O2:H2O solution with composition 1:9:1. In the first part etch rates and morphologies of etched structures were evaluated in order to characterize the anisotropy. The second part was devoted to numerical simulations of 3D etching shapes. For this purpose the database of the simulator TENSOSIM was determined. Comparison of theoretical 3D etching shapes with experimental shapes of micromachined structures supports the validity of the proposed database. So the simulator TENSOSIM is a convenient CAO tool for the design of new GaAs MEMS.