Low carbon 12Cr steel used as a nuclear steam turbine blade material have been surface-modified by the flame hardening process and the properties of hardness, hardening depth, and residual stress have been studied. For this, the thermal cycles of 12Cr steel have been controlled precisely as a function of the surface temperature, the exposed height from the water surface, and the cooling rate. The application of flame hardening increases the hardness of 12Cr steel (base value, 250 HV) to 420~550 HV considerably, forming relatively sharp hardening boundaries. Both the hardening depth and the austenitization period are linearly correlated well within the range of processing conditions employed. The rapid cooling rate also increases the hardening depth as well as the surface hardness. The properties of residual stress are discussed in terms of contributions of both the thermal and transformation stress components. The optimum processing conditions for the desirable residual stress state in the material are as follows : less than 1200℃ for the 18 mm-exposed and less than 1150℃ for full-exposed conditions by the criteria of Siemens AG-KWU, and on the basis of the specification of GE Power Generation Engineering, 944~1050℃ for the 10 mm-exposed, 883~1150℃ for the 14 mm-exposed, 833~1134℃ for the 18 mm-exposed, and 785~1097℃ for the full-exposed.