The composites of monoclinic Li3−x Na x V2(PO4)3 (x =0, 0.03, 0.05 and 0.07) and carbon are prepared by a sol–gel method. The composites are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electrochemical measurements. XRD results show that the cell volumes of the monoclinic Na-doped Li3V2(PO4)3 samples are larger than that of Li3V2(PO4)3. All Na-doped composites exhibit better electrochemical performance than that of pristine one and the Li2.95Na0.05V2(PO4)3/C composite displays highest capacity and best cycle performance. The Li2.95Na0.05V2(PO4)3/C composite presents initial capacities of 187 and 173.1mAhg−1 at 0.2C and 1C between 3.0 and 4.8V, respectively, much higher than that 175.5mAhg−1 (0.2C) and 153.4mAhg−1 (1C) of Li3V2(PO4)3/C composite. The capacity retentions of Li2.95Na0.05V2(PO4)3/C are 95.3% and 91% at 0.2C and 1C after 30 cycles, respectively, while the capacity retentions of Li3V2(PO4)3/C are 90.4% and 87% at 0.2C and 1C after 30 cycles, respectively. The rapid Li+ diffusion due to the doping of Na+ is responsible for the good electrochemical performance of Na-doped Li3V2(PO4)3 cathode materials.