This paper presents a novel one-step method for preparing of copper nanofluids by reducing CuSO(4).5H(2)O with NaH(2)PO(2).H(2)O in ethylene glycol under microwave irradiation. Nonagglomerated and stably suspended Cu nanofluids are obtained. The influences of CuSO(4) concentration, addition of NaH(2)PO(2), and microwave irradiation on the reaction rate and the properties of Cu nanofluids were investigated by transmission electron microscopy, infrared analysis, and sedimentation measurements. It is found to be a fast, efficient one-step chemical method to prepare Cu nanofluids.