Abstract: Thermography is considered as an efficient tool in detecting the defects of most kinds of materials. But with the higher requirements of NDT, especially for the aviation materials such as minuteness defects and fatigue, it becomes a challenge to every NDT methods. It has to enhance its ability in detecting the faintness heat sources. Lock-in thermography is a sensitive tool to overcome these difficulties. The basic principles, realization and key points of lock-in thermography were discussed.