The diffusion of La in ε-Fe2～3N(ε)and γ′-Fe4N (γ′)phases pro duced by plasma nitriding was studied. It is found that with increasing nitridin g time, the ε phase continuously decomposes into γ′ one under employed experi mental conditions and compared with γ′ phase or ε+γ′ ones, the mono one ε has an impedient effect on depth of La diffusion. The growth of La layer in dua l phases and mono one follows an approximate exponential law and a parabolic one , respectively, and this kinetics law does not change with increasing temperatur e. The effect of increasing temperature in the range of 520 to 560 ℃ on La d iffusion depth is less than that of phase constitution change such as the decomp osition of ε phase into γ′ one.