A terahertz (THz) wave reflection imaging system based on backward wave oscillator (BWO) was presented. It is a new approach to non-destructive testing. The intensity information of the terahertz wave after being reflected from the sample surface or substrate was collected by a pyroelectric detector, and then imaged by computer. A number of samples which had potential imaging applications were tested by using 0.7 THz radiation, including coins, badges, a model air-plane and some industrial samples that contained pre-built defects. The experimental results reveal that THz radiation is highly transparent to many industrial materials, especially the “microwave-absorbing” material which is widely used in aero and space technologies.