As an emerging non-destructive detection technology, infrared fault diagnosis is developed rapidly in recent years. Compared with the traditional methods, infrared fault diagnosis has a series of advantages such as noncontact and high sensitivity. It is being widely used in the field of fault diagnosis. For the quantitative detection of defects, people has proposed many different methods and established the theoretical model for determining the relation of the defect size to its brightness in infrared thermal images. Each method has its shortcomings and limitation. There is still no a common theoretical model for the quantitative detection of different defects so far.