Grade 91 steel has been used in nuclear and fossil power plants since the 1970s. Manufacturing variabilities resulting from manufacturing, repair, and management activities have been attributed to lowered creep and fatigue life. This paper characterizes the elastic, thermal, and anelastic properties of P91 steel with different microstructures. Eight different microstructural conditions were identified as acceptable, gross, and gradual degradations. Ultrasonic testing was used to measure velocities, and resonant ultrasound spectroscopy was used to measure internal friction. The thermal diffusivity was measured along with Vicker’s hardness and grain size. A model for internal friction was used to combine the measured elastic and thermal properties. The results suggest that the current understanding of internal friction and its sources may be incomplete for complex microstructures like grade 91. From an nondestructive evaluation perspective, the results suggest that the internal friction has the highest sensitivity to microstructure changes, compared to elastic and thermal properties.