The remaining life estimation method based on creep deformation behavior was developed in this study. Constant load creep tests in Cr-Mo-V steel was used for 91600 h experienced start/stop operation and 131 times in a turbine rotor were performed at 773 K, 823 K and 873 K in air. Creep strain curve was approximated by the modified theta-projection method, which was proposed by K. Maruyama et. al. as the following equation : ε=ε0+A{1-exp(-αt)}+B{exp(αt)-1}. A determination method for cut-off strain was proposed in this study and it is 2% offset strain due to stationary creep. From this investigation, the following conclusions were obtained. (1) Modified theta-projection method was able to express the entire creep region from primary stage to tertiary stage except just before the rupture of the material. (2) Coefficients A and B were invariable with tested conditions. Constant a was related to applied stress, tested temperature and degree of damage. It was found that the α is related to the vickers hardness of the material. (3) Estimation of creep curves up to the tertiary stage was possible from the knowledge of vickers hardness of the material, and remaining lives under actual conditions were estimated on the basis of the creep curve prediction.