Abstract: Low carbon steel was surface modified by the quench-polish-quench salt bath treatment process (QPQ). The modified surface was systematically characterized. The results reveal that the nitrided sample surface consists of Fe₃N/Fe₂₄N₁₀/FeN_0.0939, while the post-oxidized sample surface consists of Fe₃O₄/Fe₂N/α-Fe. The Fe₃N in the outer-layer and Fe₂₄N₁₀/FeN_0.0939 in the sub-layer were transformed as Fe₃O₄/Fe₂N and α-Fe, respectively, after conducting post-oxidation process. After post-oxidizing, many cracks and pores were observed on the outer-layer and sub-layer. The hardness profile of post-oxidized sample decreased about 50~100 HV_0.025 with comparison to nitrided sample. The polarization curves results reveal that the corrosion current density (i_corr) of post-oxidized sample decreased about 0.055 μA.cm^-2 with comparison to nitrided sample, indicating an improvement of corrosion resistance. However, the corrosion potential (E_corr) decreased about 79 mV, which may be caused by the cracks and pores formed on the compound layer by post-oxidizing process.