In-situ observation of fatigue crack growth behavior under ΔK-constant cyclic loading in Paris regime for two Thermo-Mechanical Control Process (TMCP) steels with different grain size was carried out. It was found from the results that crack closure dominantly contributed to fatigue crack growth resistance in the fine grain material, while crack tip stress shielding induced by crack deflection and crack branching dominantly contributed to fatigue crack growth resistance in the coarse grain material. The electron back scatter diffraction (EBSD) analysis revealed that crack deflection and crack branching in the coarse grain material were induced in the grain with (101) crystal orientation intensively under co-existence of the grain with (001) crystal orientation and also along the grain boundary between the grains with (101) and (001) crystal orientations. Hence, higher content ratio of the grains with (001) crystal orientation could contribute to higher crack growth resistance.