The early deformations in materials such as creep, plasticity and fatigue damages have been proved to have a close relationship with the nonlinear effect of ultrasonic waves propagating in them. In the present paper, the authors have proposed a theoretical mesoscale model of an ultrasonic nondestructive method to evaluate creep deformed states based on nonlinear Lamb waves. The model is presented for describing the relationship between the microstructures evolution due to creep damage and the generation of second-harmonic of ultrasonic Lamb waves propagation in metallic alloys, which is shown to depend dominantly on the dislocation density, pinning dislocation length, internal stress due to the coherency strain, volume fraction of the precipitates and the phase matching degree between the primary Lamb wave and the Double Frequency Lamb Wave (DFLW). For verifying the validity of the model, experiments were carried out in titanium alloy Ti60 plates with different residual strains, which were fabricated in a temperature of 600°C and a stress of 350 MPa with various loading time intervals. Microscopic images analyses were also performed to interpret the variation of the measured acoustic nonlinearity and to obtain the microstructure parameters of the Ti60 specimens, as shown in Fig 1. The mesoscale model was then applied to these creep damaged Ti60 specimens, which revealed a good accordance with the measured results of the nonlinear Lamb waves, as shown in Fig 2. These results indicate that the increases of the precipitation volume fraction and the dislocation density make a clear contribution to the increase of the acoustic nonlinearity of guided wave in the early creep stage, and as the creep loading progresses, the reduction of the precipitation volume fraction and the dislocation density and the increasing mismatch of the phase velocities cause a gradual decline of it in the late stage.