Abstract: The inner structures and characters of 0.2 THz imager and 0.38 THz imager is reported in this paper. Nondestructive detection of insulation foam defects utilizing the two systems is reported and discussed respectively. The results show that the image obtained with the 0.38THz system is clearer, and resolution is better with the 0.38 THz system as well. Therefore the 0.38 THz system can be widely used in quality monitoring and materials inspections.