The etching mechanism of MgO thin films in Cl-2/Ar plasma was investigated. It was found that the increasing Ar in the mixing ratio Of Cl2/Ar plasma causes nonmonotonic MgO etch rate, which reaches a maximum value at 70%Ar+30%Cl-2. Langmuir probe measurement showed the noticeable influence of Cl-2/Ar mixing ratio on electron temperature and electron density. The zero-dimensional plasma model indicated monotonic changes of both densities and fluxes of active species. At the same time, analyses of surface kinetics showed the possibility of nonmonotonic etch rate behavior due to the concurrence of physical and chemical pathways in ion-assisted chemical reaction. (C) 2004 American Institute of Physics.