In this study, the water drop impact erosion properties of 12Cr steel surface-hardened by the flame hardening process have been studied. For this, both the maximum erosion depth de,max and volume loss Ve with the number of cumulative impacts n have been investigated for the flame-hardened 12Cr steels with different hardnesses. Typically all the samples showed an erosion-time characteristic involving an incubation period initially followed by a steady state period. Compared to those for the as-received 12Cr steel, the flame-hardened ones showed an excellent erosion resistance to water drop impacts, showing a 2.2~2.8 times higher incubation time ti and 5~8 times lower erosion rate α. In the incubation period the as-received 12Cr steel was deformed by a ductile depression and ploughing, while the flame-hardened one by fatigue cracks and a brittle platelet deformation. In the steady state period the damage was progressed by a cleavage fracture for both the stages.