In the present study, the influence of the microstructure of materials on the characteristics of small-crack growth was investigated, and the minimum crack length to which the small-crack growth law (dl/dN∝l) is applicable was evaluated in the cases of ferrite-pearlite steel, ferrite-martensite steel and quenched and tempered carbon steel. Initial crack growth is influenced markedly by the grain boundary, pearlite structure and martensite structure, and these influences decrease with increase in crack length. However, even in the process in which crack growth is influenced by the microstructure, the mean crack growth rate is estimated by extrapolating the crack growth rate in the region where it is mainly controlled by the mechanical parameter alone.