The influence of change in inner layer thickness of a composite circular tube is investigated on second-harmonic generation (SHG) by primary circumferential ultrasonic guided wave (CUGW) propagation.Within a secondorder perturbation approximation,the nonlinear effect of primary CUGW propagation is treated as a second-order perturbation to its linear response.It is found that change in inner layer thickness of the composite circular tube will influence the efficiency of SHG by primary CUGW propagation in several aspects.In particular,with change in inner layer thickness,the phase velocity matching condition that is originally satisfied for the primary and double-frequency CUGW mode pair selected may no longer be satisfied.This will remarkably influence the efficiency of SHG by primary CUGW propagation.Theoretical analyses and numerical results show that the effect of SHG by primary CUGW propagation is very sensitive to change in inner layer thickness,and it can be used to accurately monitor a minor change in inner layer thickness of the composite circular tube.