Lock-in infrared thermography method was gradually being used in fatigue studies because of its advantages such as real-time, quick-reaction, non-contact, non-destructive and so on. In this paper, non-destructive testing was applied to fatigue specimen with defects, based on lock-in infrared thermography. In parallel, the result was analyzed by using lock-in infrared thermography system developed by Cedip in French. The results show that more information of internal detects can be found from phase image than that from amplitude image. The experiment procedure indicated that a proper testing frequency was the key to the non-destructive testing. The data revealed that deeper depth and larger area of defect led to a precise testing result.