Abstract: Electromagnetic guided waves theory was applied in microwave nondestructive testing, which was advanced by this paper. By building up a resonance model for the microwave signals propagating in the pipe, and by analyzing the resonance results of the microwaves at the receiving port, we derived a nondestructive method that was able to determine the degree of the wall thinning in the pipe remotely. In this paper, wall thinning having values of 3% to 80% of pipe thickness were detected significantly. In the experiment, a microwave network analyzer was employed to generate microwave signals propagating in the pipe where the frequency was swept from 47.38 to 47.47 GHz. It indicated that high precision for quantitative evaluation could be achieved by using this nondestructive method.