In this study, to evaluate the structural change due to creep degradation in P91 steels nondestructively by means of the micromagnetic multiparameter microstructure and stress analysis (3MA) device, information of dislocation density change is extracted from micromagnetic properties. Micromagnetic properties of the creep and thermal aging samples are measured by 3MA device. The relationship between measured micromagnetic properties and the low-angle grain boundary length of the samples quantified as the alternative value of dislocation density by Electron Back Scatter Diffraction (EBSD) equipment are investigated. Change of dislocation density due to creep degradation can be evaluated by some micromagnetic properties which are independent on precipitation change.