The effect of specimen geometry, specimen thickness, stress ratio and frequency on the fatigue crack growth rate of structural steels was examined at 123K to clarify the formation mechanism of cleavage during fatigue crack growth, and the condition of the final fracture. Fatigue crack growth rate at 123 K was found to be sensitive to the stress ratio, and was considerably increased by the occurrence of cleavage during striation formation. The value of Kci at which the first cleavage appears was about 20 MPa√(m) regardless of specimen configuration, stress ratio or frequency, except for the specimen which was 4 mm thick. The presence of cleavage during crack growth may be related to local deterioration of the material caused by cyclic straining ahead of the crack tip. The value of Kfc at which final fracture occurs was found to increase as the specimen thickness did not satisfy the size requirement for small scale yielding.