The fatigue crack growth rate of carbon nanotube (CNT)/graphene nanoplatelet (GNP)/epoxy composites were experimentally analyzed to study the synergetic effect of the two nano-fillers on the fatigue crack behavior of the studied composites. In the experimental program, The content of two fillers of all the nano-composite specimens was controlled to be 0.4 wt%, and five allocations between the two fillers (CNTs : GNPs =1:9, 3:7, 5:5, 7:3, and 9:1) were considered in the preparation of specimens. The compact-tension (CT) specimens were employed to investigate the fatigue crack growth rates of the composites with various filler allocations. Experimental results show that the specimens with the CNT/GNP allocation of 1:9 have the lowest fatigue crack propagation rate than those with different filler allocations, only one-tenths of that of neat epoxy. Examining the fracture surfaces of the specimens reveals that the composites with the CNT/GNP allocation of 1:9 have the roughest surfaces than those with the other filler allocations. The fillers with three-dimensional structures composed by integrating the two nano-fillers were found to deflect the direction of fatigue crack fronts effectively, reducing the crack propagation rate significantly.