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In order to investigate the effect of initial microstructure on warm deformation behavior, some specimens of 45 steel were annealed and some quenched. Then the specimens were isothermally compressed on a Gleeble 3500 machine. The deformation temperature range was 550 to 700 ℃ and the strain rate range was 0.001 to 0.1 s-1. An optical microscope (OM) and a transmission electron microscope (TEM) were used to study the microstructures. The results show that the microstructure of annealed specimens is ferrite and pearlite and that of quenched specimens is martensite. The flow stress of quenched specimens is higher than that of annealed ones at 550 ℃ when strain rates are greater than 0.001 s-1. However, at 600 to 700 ℃ and strain rate of 0.001 s-1, the whole flow curves of quenched specimens are below that of annealed ones. Under the rest conditions, the flow stress of quenched specimens is higher at the beginning of compression and then the opposite is true after the strain is greater than a critical value. The microstructure examination proves that the tempering and dynamic recrystallization easily occur in the specimens with martensite during warm compression, which results in the above phenomena.