A one step method for the fabrication of Mn3O4 nanoparticles/graphene composite materials is described. An in-situ transformation technique using an aqueous phase at ambient conditions is employed. During the process of forming Mn3O4 nanoparticles, graphene oxide is simultaneously reduced to a graphene nanosheet which simplifies the overall process and eliminates the need for an additional reduction step, which is generally energy consuming and produces toxic by-products. The resulting materials were characterized using XPS, XRD, TEM, STEM–EDS and electrochemical analysis. The Mn3O4 nanoparticles with uniform size (about 14 nm) and shape are highly dispersed on the surface of graphene. The material exhibits a reversible capacity of over 500 mAh g−1, at a current density of 60 mA g−1 with less significant fades after 40 cycles. Furthermore, its rate capability and cycling stability are enhanced in comparison with that of the pure Mn3O4 nanoparticles.