The oxygen tracer diffusion coefficient in high‐purity polycrystalline was determined using as a tracer. The specimen was prepared by annealing powder at 1400°C. The diffusion process was monitored by reaction gas analysis or by the microbalance method. Within the range of 800 to 1100°C, the oxygen‐tracer diffusion coefficient. (D*) was calculated to be . This is smaller than the value reported in the literature for . The smaller value is thought to be due to the lack of an oxygen concentration gradient in the present study.