The diffusion of La in ε-Fe 2～3N(ε)and γ′-Fe 4N (γ′)phases produced by plasma nitriding was studied. It is found that with increasing nitriding time, the ε phase continuously decomposes into γ′ one under employed experimental 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 dual phases and mono one follows an approximate exponential law and a parabolic one, respectively, and this kinetics law does not change with increasing temperature. The effect of increasing temperature in the range of 520 to 560 ℃ on La diffusion depth is less than that of phase constitution change such as the decomposition of ε phase into γ′ one. The diffusion of La in ε-Fe 2 ~ 3N (ε) and γ’-Fe 4N (γ ’) phases produced by plasma nitriding was studied. It is found that with increasing nitriding time, the ε phase is decomposed into γ’ one under employed experimental 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 dual phases and mono one follows an approximate exponential law and a parabolic one, respectively, and this kinetic law does not change with increasing temperature. The effect of increasing temperature in the range of 520 to 560 ° C on La diffusion depth is less than that of phase constitution change such as the decomposition of ε phase into γ ’ one.