MgO.$Al_2O_3$ polycrstalline spinel powder was subjected to vaporization over the temperature range of 1150-130$0^{circ}C$ under H2 atmosphere. Diffusion coefficient of oxygen ion through the spinel were calculated using the measure vaporization rates as follows : D=28.4 exp(-901500/RT) Reference data of the vaporization rates of MgO.$8Al_2O_3$ single crystal spinel were applied to the vaporiza-tion model proposed in this study and were calculated to give the oxygen ion diffusion coefficients over the tempera-ture 1700-195$0^{circ}C$. The obtained diffusion coefficients are as follows: $D=3.20{ imes}106$ exp(-155600/RT)