Metal materials especially steel or cast iron are used extensively in many types of construction such as buildings, bridges, cranes, vehicles and so on, so detection or prediction of metal failure is very important, but deformation of metal material often deliveries heat energy which can be detected by infrared imager. The test specimen is installed between the two grips of the testing machine and then loaded in tension and in compression. The IR913A infrared imager is used to observe the deformation of metal specimen. The high-sensitive infrared thermal images of metal specimen in the different phase of deformation are obtained. The paper theoretically analyzes the reason from the thermodynamics and plastic mechanics, the conclusions are drawn as follows: 1) When applying loads, temperature field on the surface of metal specimen is changing, the local rise of temperature is remarkable, this can be observed from the infrared thermal images. 2) From the infrared thermal images, rising temperatures are found in the regions of stress concentrations. 3) When the test specimen approaches to failure or appears fracture, there is a remarkable change that can be shown from the infrared thermal images with remarkable colors.