The transition of the creep deformation mechanism in the low-stress region of Grade P92 high Cr ferritic heat-resistant steel was investigated by a helicoid spring creep test. Specifically, the effect of variation in the microstructure of steel on creep deformation behavior was evaluated by subjecting samples to thermal aging for 1000, 3000, 5000, and 10000 h at 700°C over a wide stress range. In addition, stress exponents were determined from the stress dependence of the minimum strain rate in the creep curves up to 270 ks. The transition of the creep mechanism was indicated when the stress exponent decreased from 4 in the high-stress region to 1 in the low-stress region below 40 MPa. A quantitative evaluation of the microstructure of a tempered martensite sample, including the determination of the amount of dissolved Mo and W, dispersion state of the precipitates, and length of the grain boundaries per unit area, was also carried out. Furthermore, the change in the minimum strain rate was evaluated as a function of the microstructural changes that accompanied thermal aging. It was found that the change in the strain rate was the most affected by the fineness of the martensitic lath structure in the high-stress region and by the dispersion density of M23C6 precipitates in the low-stress region. Based on these results, it was concluded that the microstructural parameter that most affects creep deformation behavior differs depending on the stress region due to the difference in the creep mechanism.