Plain low-carbon steel with 0.2 wt % carbon (steel 20)was first treated with plasma nitriding and then was processed by salt bath chromizing at 500 ℃ for 1h, 5hs and 12hs, respectively. The microstructure of chromized layers was studied by means of optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results have shown that the chromized layer consists of CrN, β-(Cr, Fe) 2 (N, C) 1-x . By prolonging the chromizing time, the amount of β-phase on the surface layer increases and Cr7C3 phase can appear on the surface. The chromized layer formed at 500 ℃ for 12 h contains both sub-micron CrN grain area and nano-material area. The fine CrN grains can grow up to hundreds of nanometer,when there are a lot of twins or/and stacking faults in the Cr7C3 grains.