Mechanical properties of Si-added middle carbon martensitic steels are strongly affected by the tempering temperature and carbides precipitation. In this work, relationships between Si contents, mechanical properties and carbides precipitation are investigated using Fe-0.55C-(0.3-2.0)Si-0.7Mn-0.7Cr steels. Carbides precipitates and microstructures are evaluated by Differential Scanning Carolimetry and Scanning Electron Microscope observation, respectively. As a result, every steel with various Si content showed temper brittleness, and temperatures of embrittlement are increased with Si content increment. DSC data indicated that condition of carbides precipitate changed at the temperature of embrittlement and the temperature of recovery from embrittlement. It is observed that carbides precipitate and grow with the shape of platelet inside the martensite blocks at the temperature of embrittlement. And it is also observed that granular carbides precipitate at the grain boundaries of the martensite blocks at the temperature of recovery from embrittlement. Based on these results, the mechanism of temper brittleness is estimated as follows; platelet carbide precipitated inside martensite blocks interacts with dislocation movement and inhibits plastic deformation, so that steel easily fractures from grain boundaries and indicates low charpy impact value.