Abstract: The evolution law of ferrite (F) and martensite (M) dual- phase microstructure of heavy truck beam steels, which were processed at different inter- critical quench (IQ) temperatures, were investigated with optical microscope and scanning electron microscope. The mechanical properties of test steel at different IQ temperatures and 520℃ tempering were investigated with tensile tests. The results show that IQ temperature has a certain influence on the content and morphology of M in the quenched structure of test steel, and the content of M increases with IQ temperature. M is distributed in the F matrix in granular or island shape from starting temperature of the austenite transition to 800℃. At the temperature of 800℃and above, M begins to become a matrix phase and its part exhibits a clear band shape. The content and properties of M in the critical zone of the test steel have an important influence on the mechanical properties of the tempering. The yield and tensile strength of IQ state test steel are positively correlated with the M content of the quenched microstructure, and while the proportional extension is negatively correlated with IQ temperature. The ratio of yield strength (YS)/tensile strength (TS) to IQ temperature is "V", and the optimum YS/TS is 0.835 when the IQ temperature is 800℃.