Lock-in infrared thermography method is gradually used in fatigue studies because of its advantages such as real-time, quick-reaction, non-destructive, non-contact and so on. Non-destructive testing is applied to fatigue specimen with defects, and the fatigue limit is detected rapidly based on lock-in infrared thermography. In parallel, the results are analyzed using lock-in infrared thermography system developed by Cedip in French. It shows that more information of internal detects can be found from phase image than from amplitude image. The experimental procedure indicates that a proper testing frequency is the key to the non-destructive testing. The datas reveale that larger area of defect leads to a precise testing result and the fatigue limit of the specimen decreases with the increase of the defect depth.