The diffusion coefficient of Fe, Cr and Co in Fe (Iron-armco and Steel with O of C) is subjected to exponential dependence in the interval of temperature from 1100-1250 degrees C and it deviates from this dependence below the 1100 degrees temperature interval. The detected deviation, from the exponential dependence, is the result of different structural condition of austenite under high and low temperatures. The purpose of this study is to verify and further confirm the above mentioned phenomenon in connection with self-diffusion of iron. It was proposed in this work to study in more detail the effect of the size of austenite grain on the coefficient of self-diffusion for iron under various temperatures.