Using DC-RF magnetron sputtering method, the stress free c- and r-oriented Cr2O3 thin films were grown on c- and r-cut sapphire substrates, respectively. The c-oriented film grown at 580 ºC shows the smoothest surface with a surface average (R a) of 0.17, although the c-surface energy is the highest. The origin of the smooth surface is expected that the presence of a twin grain due to a dislocation of Cr atoms, demonstrated by a reciprocal space mapping. The step height corresponding to that of the bulk is clearly observed. The r-oriented films epitaxially grow without twin grain. The R a is 1.56 in the film grown at 580 ºC because of deep trenches due to a lattice mismatch and no dislocation like c-oriented films. Since the surface energy of the r-surface is the lowest, the terrace is quite smooth in one grain even at higher substrate temperature of 840 ºC.