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To determine the effects of microstructure on the fatigue crack propagation behavior in SM45C steel, experimental investigations have been carried out with the specimens of different grain size and different spacings between the Fe3C particles. Fatigue and tensile tests were conducted and the quantitative analysis of the relations between the threshold stress intensity range and grain size, inter-particle spacing and yield stress were carried out. Some of the conclusions obtained are as fallows; (1) ΔKth was observed to increase with grain size and the spacing between the Fe3C particles. (2) In both pearlite and spheroidite microstructures, ΔKth was increased when yield stress was decreased.