Abstract: To satisfy the requirement for defect quantification by lock-in thermography, a defect quantification method based on fuzzy C-means (FCM) and edge detectors was proposed. A finite element simulation model for the lock-in thermography inspection of prefabricated flat-bottom hole was built to find out the optimal modulated frequency. Experiments conducted on the specimen using lock-in thermography testing system with the optimal modulated frequency show that most of quantitative results of FCM-edge detector method with region division are better than that of the shearing-phase method, which is widely applied in lock-in thermography. The image processed by FCM-edge detector method contains much more information about defect edge than results got by shearing-phase method. Therefore, through appropriate image processing methods, area quantification is able to be achieved with FCM-edge detector method. The research results show that FCM-edge detector method with region division has good prospects in quantitative determination of defect by lock-in infrared thermography.