Abstract: The diffusion couple was used to study solid state reaction in the Fe_xO-TiO₂ system at a temperature range of 1323 to 1473 K,during which the oxygen partial pressure was controlled by CO-CO₂ gas mixture. The cross-sections of the diffusion couple were observed by electron probe microanalysis(EPMA),and the diffusion concentration profiles of Fe²⁺,Ti⁴⁺,and O^2- ions were also determined. In combination with the phase diagram of the Fe O-TiO₂ system,it was found that ulvospinel,ilmenite,pseudobrookite phases formed during the reaction and their growth was diffusion-controlled. According to the concentration changes of diffusion ions,the diffusion path of Fe ions was plotted,based on which the solid state reaction mechanism of this system was reasonably proposed.