The redox (Reduction-Oxidation) flow battery is one of the most promising rechargeable batteries due to its ability to average loads and output of power sources. The transient characteristics are well known as the remarkable feature of the battery. Then it can also compensate for a sudden voltage drop. The dynamics are governed by the chemical reactions, fluid flow, and electrical circuit of its structure. This causes the difficulty of the analysis at transient state. This paper discusses the transient behavior of the redox flow battery based on chemical reactions. The concentration change of vanadium ions depends on the chemical reactions and the flow of electrolysis solution. The chemical reaction rate is restricted by the attached external electric circuit. In this paper, a model of the transient behavior is introduced. The validity of the derived model is examined based on experiments for a tested micro-redox flow battery system.