Abstract: A two-dimensional heat transfer model is built and solved with finite element method to study temperature distribution on outer surface of a test piece with a self-heating subsurface defect. Based on conjugate gradient method, an algorithm is proposed for identification of subsurface defect with temperature on the outer surface of the teat piece measured with an infrared imager. Effectiveness of the method is validated by numerical experiments. It is concluded that the method identifies serf-heating defects with high precision for test pieces with small thermal conductivity. Effect of temperature measurement error, number of temperature measurement point and initial defect boundary guess on defect identification can be neglected. The greater the maximum temperature difference on outer test piece surface is, the higher precision of identification is.