The solid-state reaction; 1/2 Y2O3 + 1/2 Cr2O3 = YCrO3 was conducted between 1458 and 1719 K in air (P-O2 = 0.21 atm). The reaction obeyed the parabolic rate law and the diffusion of yttrium ion controlled the reaction. According to Wagner’s theory, the diffusion coefficient of yttrium ion Y3+ in YCrO3 was evaluated using the Gibbs free energy change of the reaction and the parabolic rate constant, The defect reaction [GRAPHICS] is predominant in the formation of yttrium vacancy. The diffusion coefficient of yttrium ion is given D-Y3+/m(2) s(-1) =[8.36 x 10(-9) exp (-272/kJ mol(-1)/RT)] P-O2(3/16) a(Cr2O3)(1/8) where a(Cr2O3) is activity of Cr2O3 and P-O2 is oxygen pressure.