In this paper, a method which can inspect a metal pipe in a large scale and measure the wall thinning remotely was demonstrated. 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. A copper pipe with inner diameter of 17 mm, 1 mm wall thickness, 900 mm length, and connected with joints having the length of 17 mm and values of thickness reductions from 3 to 80% wall thickness was measured. By building up a resonance model for the microwave signals propagating in the metal pipe, and by analyzing the resonance results of the microwave signals at the receiving port, a nondestructive method was derived to determine the degree of the wall thinning in the pipe. By comparing the experimental results with the evaluation results obtained by the theoretical analysis, it is shown that the errors of the evaluation are less than 0.5% of the pipe diameter. It indicates that high precision for remote detection and quantitative evaluation of wall thinning can be achieved by using this nondestructive method.