The fatigue crack growth behavior in 20CrMnTi lath martensitic steel with a hierarchical structure was investigated at room temperature. The microstructure was quantitatively characterized by optical microscopy, scanning electron microscopy, electron backscattering diffraction, and transmission electron microscopy. Crack growth tests were performed at a stress ratio of 0.1 under the control of a stress intensity factor range using compact tension specimens. The results show that the lath martensite with medium grain size quenched in ice salt water possessed the highest fatigue crack propagation resistance, which was attributed to the relatively high intrinsic propagation resistance and extrinsic propagation resistance. Meanwhile, the results indicate that the intrinsic propagation resistance increased with the grain size, while the extrinsic propagation resistance showed an inverse tendency. In addition, the effect of substructure on the fatigue propagation properties is discussed. The deflection angle of block interfaces in the crack growth path indicated the block unit had a significant effect on the fatigue crack propagation behavior of lath martensitic steel. Based on the Taylor model, the block size (db) was considered to be the effective grain size for controlling the crack growth threshold value (∆Kth).