In order to correlate macroscopic low-cycle fatigue laws such as the Manson-Coffin relation and Miner's rule with the actual microscopic fatigue process, cyclic strain-controlled low-cycle fatigue tests were conducted on a medium carbon steel under constant-, two-step-, and random-strain amplitude conditions. Through continual observation of the microscopic fatigue process of plain specimens and holed specimens with a very small hole (40 mu m diameter), it was found that the total fatigue life is mostly occupied by the microcrack propagation life. Both the Manson-Coffin relation and Miner's rule are derived using the microcrack propagation law. Then, background for the applicability of Miner's rule is discussed. The results of the random loading fatigue tests showed that microcrack propagation life can be predicted on the basis of the microcrack growth law in conjunction with the rain flow method for counting fatigue damage. Finally, the microcrack growth law-aided approach was applied to interpret anisotropy in fatigue strength of a thick forged steel plate.