The application of epoxy adhesive joints in structural components fabricated by fiber-reinforced composites has increased significantly in recent years. However, the degradation of adhesively bonded joints through the effects of moisture is one of the major concerns affecting their wide implementation in composites. Therefore, non-destructive evaluate (NDE) method is necessary to detect the degradation of the adhesive. In this study, the pulse type terahertz time domain spectroscopy (THz-TDS) system was devised and used for evaluating the moisture absorption effects in epoxy adhesive-bonded glass fiber reinforced composite single lap joints. Tensile shear test was performed to estimate the degradtion of adhesive strength on sigle lap joints depending on moisture absorption time. Also, the complex refractive index was measured by THz-TDS system to correlate with degradation of adhesive strength. From these results, the degradation of adhesive strength on composite single lap joints depending on moisture absorption was evaluated by change of complex refractive, non-contact measured by THz-TDS system. In conclusion, it is expected that THz-NDE technique can be widely used to evaluate the reliability of composite structures.