The feasibility of using the nonlinear effect of primary circumferential guided wave (CGW) propagation for char-acterizing the change of inner layer thickness of a composite circular tube (CCT) has been investigated.An appropriate mode pair of the fundamental and double-frequency CGWs (DFCGWs) has been selected to enable the second harmonics of primary wave mode in the given CCT to accumulate along the circumferential direction.When changes in the inner layer thickness (described as the equivalent inner layer thickness) take place,the corresponding nonlinear CGW measurements are conducted.It is found that there is a direct correlation between change of equivalent inner layer thickness of the CCT and the relative acoustic nonlinearity parameter (Δβ) measured with CGWs propagating through one full circumference,and that the effect of second-harmonic generation (SHG) is very sensitive to change in the inner layer thickness.The experimental result obtained demonstrates the feasibility for quantitatively assessing the change of equivalent inner layer thickness in CCTs using the effect of SHG by primary CGW propagation.