Experimental tests have been carried out in order to investigate the influence of the heat treatments on the microstructures and mechanical properties of FV520B stainless steel. The fatigue strength of the common and heat-treated FV520B steels was rapidly assessed using the lock-in infrared thermographic methodology. Good agreements were achieved between the predicted values and those obtained by the traditional testing procedures. The temperature increments during the fatigue tests were tightly linked with the internal microstructural changes. The features of the fatigue fracture surface were observed by the scanning electron microscope. It is found that the fatigue microcracks were initiated from the corner of the specimen surface. The metallographic analysis shows that the improvement of the mechanical properties of FV520B steel is attributed to the formation of the fine-tempered martensite along with the secondary phase particles uniformly distributed at the grain boundary.