Li3V(2−2x/3)Mg x (PO4)3/C (x =0, 0.15, 0.30, 0.45) composites have been synthesized by the sol–gel assisted solid state method, using adipic acid C6H10O4 (hexanedioic acid) as carbon source. The particle size of the composites is ∼1μm. During the pyrolysis process, Li3V(2−2x/3)Mg x (PO4)3/C network structure is formed. The effect of Mg2+ doped on the electrochemical properties of Li3V2(PO4)3/C positive materials has been studied. Li3V1.8Mg0.30(PO4)3/C as the cathode materials of Li-ion batteries, the retention rate of discharge capacity is 91.4% (1C) after 100 cycles. Compared with Li3V2(PO4)3/C, Li3V(2−2x/3)Mg x (PO4)3/C composites have shown enhanced capacity and retention rate capability. The long-term cycles and ex situ XRD tests disclose that Li3V1.8Mg0.30(PO4)3 exhibits higher structural stability than the undoped system.