Vanadium-substituted LiFe0.5Mn0.5SiO4 cathode material for Li-ion batteries is synthesized by solid state reaction, and the effect of V-substitution on the material is investigated by X-ray diffraction and charge–discharge tests. X-ray diffraction patterns seem to indicate that V+5 is substituted for Si4+ in Li2+xFe0.5−x/2Mn0.5−x/2Si1−xVxO4 (x = 0, 0.1, 0.2, 0.3, 0.4) by maintaining electroneutrality. The substitution of V for Si in Li2Fe0.5Mn0.5SiO4 increases the charge and discharge capacities as a cathode material of Li-ion batteries; Li2.2Fe0.4Mn0.4Si0.8V0.2O4 delivers an initial discharge capacity of 197 A h/kg, while Li2Fe0.5Mn0.5SiO4 delivers that of 134 A h/kg. The main reason for the high capacity with the V substitution for Si in Li2Fe0.5Mn0.5SiO4 is considered not due to the redox capacity of V but due to the lowering of the charge voltage, while, substituting a higher amount of V, with x over 0.2 in Li2+xFe0.5−x/2Mn0.5−x/2Si1−xVxO4, reduces capacity in accordance with theoretical capacity calculation. In order to confirm the V-substitution site exactly, however, strong evidence by Rietveld refinement is necessary, which is left as our urgent future work.