Ni(x)Co(100-x) (x = 0, 25, 50, 75, and 100) nanoparticles were uniformly in situ grown on reduced graphene oxide (RGO) nanosheets by a coreduction process for the first time. The as-synthesized products were characterized by X-ray powder diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma optical emission spectrometry (ICP-OES), and transmission electron microscopy (TEM). It was found that RGO nanosheets can effectively prevent the aggregation of Ni(x)Co(100-x) nanoparticles. The size and morphology of the Ni(x)Co(100-x) nanoparticles on RGO nanosheets can be slightly adjusted by changing the Ni:Co atomic ratio. The magnetic properties of the RGO-Ni(x)Co(100-x) composites were investigated at 300 and 1.8 K, respectively. The results reveal that the composites have ferromagnetic characteristics and show composition dependent magnetic properties. In addition, these RGO-Ni(x)Co(100-x) nanocomposites also exhibit enhanced catalytic activities toward the reduction of 4-nitrophenol (4-NP) by NaBH(4) as compared with bare Ni(x)Co(100-x) alloy, and the RGO-Ni(25)Co(75) shows the highest catalytic activity among the obtained nanocomposites. This general and facile coreduction route can be extended to synthesize other alloy nanostructures on RGO nanosheets with various morphologies and functions, and provides a new opportunity for the application of graphene-based materials.