Abstract: The materials investigated were a 40Si2Mn2Mo steel and two kind of 40Si2Mn2Mo steels with different molybdenum content. By means of TME and physicchemical phase analysis technique, it has been found that there exists a transition from e-carbide to cementite in the tempering temperature range corresponding to TME in 40Si2Mn2 steel.Nevertheless, the cementite precipitated at this stage of tempering is not platelets as generally observed in other steels, but is fine dispersive particles. The correspondence between the fine dispersive carbide and TME is confirmed in 40Si2Mn2Mo steels. It ha.s been concluded that a large amount of line dispersive cementite precipitating during tempering is a primary factor for TME in medium carbon Si-Mn steels.