Abstract: Any factors affecting the microstructure would cause the magnetic Barkhausen noise change, therefore, a new method to detect carbon content of ferromagnetic materials based on Barkhausen signal was proposed in this paper. The annealed carbon steels were used to investigate the dependence of hardness and MBN signal on carbon content. The hardness increases with the increasing carbon content, and the characteristic values of the MBN increases initially and then decreases with the increase of carbon content, reaching a maximum value at 0.55%. The 1045 carbon steels with different hardness were obtained by treating with different tempering temperatures, finding that the MBN decreases gradually with the increasing hardness. By contrast, the effect of carbon content on the MBN is not only caused by hardness, but also by carbon content itself. When the carbon content of the sample to be measured is in the range of 0.32%~0.55% or 0.55%~0.80%, the MBN signal increases or decreases monotonically with the carbon content, and the carbon content can be measured directly by the MBN signal. When the carbon content is between 0.32% and 0.80%, the MBN measurement and hardness measurement can be combined to achieve a nondestructive evaluation of carbon content.