The hot corrosion behavior of a directionally solidified Co-base superalloy in different mediums were investigated by using resistance furnace and SEM methods. As showed by the testing results, the most severity corrosion occurred in Na2SO4, and the least in 75%Na2SO4+25%NaCl when the alloy was corroded at 700℃ for 10 hours.Analysis of SEM and XRD showed that Al2(SO4)3,Cr2Ni3,Cr4Ni15W and Al4CrNi15 formed which indicate acid melting and fusing reaction occured when the alloy corroded in Na2SO4, oxides and chlorides indicate that active oxidation reaction when corroded in NaCl.For corroding in 75%Na2SO4+25%NaCl, some Ni-S compounds formed and the reactions mentioned above were restrained.