Four modified Al diffusion coatings(Al,Cr-Al,Al-Si and Cr-Al-Si coatings) were prepared on Ni_3Al based single crystal superalloy IC20.The oxidation tests were carried out at 1 150℃for up to 100 h.Cyclic hot corrosion tests were carried out at 950℃for 50 h.The results indicate that the oxidation and corrosion resistances of IC20 alloy are improved significantly by the coatings,and both oxidation and hot corrosion resistances of the four coatings are rated in the order(from worst to best) of Cr-Al,Al, Cr-Al-Si,Al-Si coatings.It is found that the degradations of Al and Cr-Al coatings are very quick due to the serious inter-diffusion between the coatings and substrates.The inter-diffusion between Si-containing coatings and substrates is reduced since Si effectively retards the outward diffusion of Mo.The weak effects of Cr and benefit effects of Si to the oxidation and hot corrosion resistance were discussed.The hot corrosion degradation mechanism of superalloy IC20 was analyzed. Four modified Al diffusion coatings (Al, Cr-Al, Al-Si and Cr-Al-Si coatings) were prepared on Ni_3Al based single crystal superalloy IC20. The oxidation tests were carried out at 1 150 ℃ for up to 100 h. Cyclic hot corrosion tests were carried out at 950 ℃ for 50 h. The results indicate that the oxidation and corrosion resistances of IC20 alloy are improved significantly by the coatings, and both oxidation and hot corrosion resistances of the four coatings are rated in the order (from worst to best) of Cr-Al, Al, Cr-Al-Si, Al-Si coatings.It is found that the degradations of Al and Cr-Al coatings are very quick due to the serious inter-diffusion between the coatings and substrates The inter-diffusion between Si-containing coatings and substrates is reduced from Si effectively retards the outward diffusion of Mo. The weak effects of Cr and benefit effects of Si to the oxidation and hot corrosion resistance were discussed. The hot corrosion degradation mechanism of superalloy IC20 was analyzed.