Inositol and phytic acid have been employed as organic additives of the positive electrolyte for all-vanadium redox flow battery (VRFB) to improve its stability and electrochemical reversibility. Thermal stability of the V(V) electrolyte could be improved by both inositol and phytic acid additives. The results of cyclic voltammetry (CV), steady polarization curve and electrochemical impedance spectroscopy (EIS) show that the electrochemical activity of the electrolyte with additives is improved compared to the blank one. The diffusion coefficient of V(IV) species with inositol has been increased from 0.71–1.16×10−6 to 3.11–5.15×10−6 cm2 s−1 and the exchange current density was raised from 2.8×10−3 to 11.7×10−3 Acm−2. Moreover, electrochemical results suggest that the positive electrolytes with organic additives have better cycling stability. The VRFB employing positive electrolyte with inositol as additive exhibits excellent charge–discharge behavior with an average energy efficiency of 81.5% at a current density of 30mAcm−2. The UV–visible spectroscopy confirms that new substances in V(V) electrolyte are not formed with both inositol and phytic acid additives.