A very efficient, in situ growth approach has been developed to fabricate a graphene oxide (GO)-supported, high-performance Ni_1−xFe_x loading electrocatalyst. Transition metal nanoparticles (NPs) such as Ni are quite attractive due to their advantages in cost, and excellent electrocatalytic activity for hydrazine electrooxidation. However, their strong aggregation propensity makes it difficult to develop them into highly efficient electrocatalysts. Polyethyleneimine (PEI), a branched polyelectrolyte, is devised to resolve this problem, and is utilized to uniformly anchor Ni–Fe NPs on GO. Meanwhile, partial GO can be reduced to reduced graphene oxide (rGO) in the presence of amine-containing molecules, which is favourable to enhance the electrical conductivity of supporting materials. Ni_1−xFe_x NPs (50% loading) are prepared by a co-reduction method of metal precursors with hydrazine hydrate under 100 °C for 3 h. The content of PEI attached on GO significantly affects the dispersion and size of the resulting Ni_1−xFe_x NPs. Transmission electron microscope (TEM) images reveal that the Ni_1−xFe_x NPs with an average size of 8 nm are homogeneously dispersed on PEI–rGO_10 : 1 sheet. The electrochemical measurements indicate that the Ni_1−xFe_x/PEI–rGO_10 : 1 hybrid with a Ni : Fe mass ratio of 80 : 20 exhibits the highest electrocatalytic activity toward hydrazine oxidation, while the PEI–rGO_10 : 1 supporting material plays an excellent role in the synergistic catalytic effect. Under the same conditions, the electrocatalytic activity of Ni₈₀Fe₂₀ NPs on PEI–rGO_10 : 1 for hydrazine is 2 times higher than that of the Ni₈₀Fe₂₀ NPs directly deposited on GO.