Radiation damage induced on a gallium arsenide surface by electron cyclotron resonance (ECR) plasma etching is studied. The number of displaced atoms (displaced atom density) formed by exposure to ECR plasma is determined quantitatively by Rutherford backscattering spectrometry aligned spectra. The density increases with increasing microwave power. The amount of increase of displaced atom density from the undamaged virgin surface &Dgr;Ddais 2.9×1015and 4.4×1015atoms/cm2at microwave power of 500 W (ion current density,Ii: 0.80 mA/cm2) and 800 W (Ii: 2.00 mA/cm2), respectively. The reflection high‐energy electron diffraction study shows that an amorphous layer is formed on the surface at 800 W (Ii: 2.00 mA/cm2). The effect of bias power and chamber pressure on the damage is also studied. The increase of damage (&Dgr;Dda: 3.2×1015atom/cm2) from ECR plasma etching with an rf bias of 15 W is less by a factor of 2 than damage of reactive ion etching (&Dgr;Dda: 6.9×1015atom/cm2) when compared at an ion current density of 1.17 mA/cm2. Low‐damage anisotropic etching can be achieved by ECR plasma etching.