The EPR parameters of three nonequivalent Er3+ centers in ThO2 have been reasonably explained by diagonalizing complete energy matrices. The results indicate that the center III may be attributed to a positive charge compensator located at the [111] axis of cube, and the nearest ligand close to the compensator has a displacement away from central ion by about 0.19 angstrom. As for the center II, it is expected that the four ligand ions close to the compensator have a displacement by about 0.046 angstrom along the [100] direction. Moreover, the dependence of EPR g-factors on shift parameter Delta Z is discussed. (C) 2011 Elsevier B.V. All rights reserved.