The present disclosure relates to a redox flow battery using an electrolyte concentration gradient, capable of increasing the efficiency of the redox flow battery, and to an operation method thereof. The redox flow battery includes a catholyte tank having an electrolyte inlet at the top thereof and an electrolyte outlet at the bottom thereof and having a partition plate for forming a concentration gradient of a catholyte received therein, an anolyte tank having an electrolyte inlet at the top thereof and an electrolyte outlet at the bottom thereof and having a partition plate for forming a concentration gradient of an anolyte received therein, and a stack for charging and discharging power by receiving the catholyte and the anolyte supplied from the catholyte tank and the anolyte tank.