Abstract Nonlinear Lamb wave was used to analyse the effect of tempering temperature on modified 9Cr-1Mo (P91) steel. As-received material was normalized at 1080°C and then tempered in temperature range of 600-850°C with a step size of 50°C for 1.5 h and followed by furnace cooling. Nonlinear ultrasonic parameter β was determined at each temperature using Lamb wave and correlated with microstructural characteristics like size of precipitate and density of dislocation. It was seen that acoustic nonlinearity parameter (β) was sensitive towards precipitate-matrix coherency strain, generated during tempering. © 2015 Elsevier Inc.