The increasing industrial application of fiber reinforced plastics demands developments of new techniques for non-destructive testing. The sub-terahertz frequency band has proved to be a noteworthy option for this task. Composite structures or laminates can be inspected for foreign inclusions, delaminations, debonds, etc., using sub-terahertz sensors during the manufacturing process or maintenance. In this contribution we present our results using a frequency modulated continuous wave terahertz imaging system in comparison with conventional NDT measurements on several different GFRP samples. Thereby a semi-automatic terahertz image processing approach for enhanced defect detection is applied.