A method is presented for monitoring the state of charge (SOC) of vanadium redox flow batteries (VRBs) by the transmission spectrum of the positive electrolyte [V(IV)/V(V)]. We use the transmission spectrum rather than the absorption spectrum for better signal noise ratio. In order to solve the complicated relations between the spectrum and the SOC of the VRB, the entire shape of the spectrum is utilized instead of the data at one wavelength. We experimentally demonstrate that SOCs from 0 to 100 % can be monitored by comparing the transmission spectra of the positive electrolytes [V(IV)/V(V)] with the database, which is previously built by detecting standard samples. The method has potential ability to monitor the level of imbalances of the VRB, which is important for rebalancing the electrolyte and restoring the capacity loss of a VRB.