Abstract: The mechanical properties of low carbon steel, middle carbon steel and high carbon steel have been studied on internal microstructure by the scanning electron microscopy in-situ tensile test. The microstructure mechanism of their mechanical properties has been analyzed. The results are: the mechanical properties of the high carbon steel dependent on the size of pearlite lump and the distance between ferrite lamella and cemetite lamella and the cemetite size and distribution. The finer the pearlite lamella, the thinner the ferrite and the cemetite, the more the boundary between the ferrite and the cemetite, the higher the mechanical properties of the higher carbon steel. The mechanical properties of the middle carbon steel dependent on the size of ferrite grain and pearlite lump. The smaller the size of ferrite grain and pearlite lump and the more dispersed distribution of the paertite made the mechanical properties of the middle carbon steel increased. The mechanical properties of the low carbon steel dependent on the grain size of ferrite and was also affected by the peartite size and distribution. The smaller the size of ferrite grain and pearlite lump and the more dispersed distribution of the paertite, the higher the mechanical properties of the middle carbon steel.