Four sets of experimental data of a minimum creep rate of P91 in different range of stress (from 1.1 to 350 MPa) were compiled for the calibration of the linear + power law and modified sine law. An investigation was conducted to examine the accuracy of both laws in different range of stress. It appears that modelling result of linear + power law fitted experimental data well in low and high stress level while modified sine law fits experimental data in all stress level. The predicted results from narrow ranges of stress indicates that linear power law requires data from both high and low stress levels to make a prediction, modified sine law could make prediction on a wider range of stress with data from a narrow range of stress. This conclusion could help research on predicting low stress level’s data with experimental data from high stress level.