The aim of this work is to characterize the microstructures of quenched and tempered steels non-destructively by a diverse set of parameters of the Magnetic Barkhausen Noise method (MBN fingerprint, frequency spectra, pulse height distribution, root-mean-square, and total number of pulses). Identical specimens from a SAE 5140 steel bar were prepared. All specimens were austenitized at 860 degrees C for 30 minutes and water-quenched identically. The quenched specimens were then tempered at various temperatures between 200 degrees C and 600 degrees C. The microstructures were characterized by metallographic examinations and hardness measurements. Pulse height distributions, noise signal envelopes and frequency spectra were used to evaluate Barkhausen activity. The results show that as the tempering temperature increases, the Barkhausen activity increases due to the enhancement of domain wall displacement with softening of the martensite. An excellent correlation was found between Barkhausen parameters and hardness values.