Etching characteristics for GaAs and InP using an ultrahigh vacuum reactive ion beam etching (UHV-RIBE) system have been classified. Using a horizontal electron cyclotron resonance (ECR) ion source, a uniform ion beam covering a large area is obtained, and etching at low pressure (≂10−5 Torr) has become possible. The etching gas Cl2, is ionized at a gas pressure of 6×10−4 Torr, and typical applied microwave power is 300 W. We have classified the etching behavior into three categories as a function of extraction voltage and tried to clarify the change of the edge profile and surface condition. It is possible to obtain a vertical mesa. The maximum etch rates for InP and GaAs are 2.0 μm/min and 1.2 μm/min at high temperature and high applied voltage. Under appropriate conditions almost comparable etch rates for GaAs and InP have been obtained.