The oxygen evolution reaction (OER) has a vital role in many alternative-energy systems because it provides the protons and electrons required for converting electricity into chemical fuels1. Metal oxides and oxyhydroxides shows state-of-the-art activity for the OER, and their reaction kinetics and mechanism are generally analyzed by phenomenological Tafel equation (applied overpotential vs. log (current density), η-log(i)) and power law rate equation2 (current density vs. oxidative charge density, i ∝Qn, where n is reaction order relative to charge density). Here it is demonstrated that none of them is utterly appropriate and that the current density equals the oxidative charge density exponential dependent rate constant multiplying the oxidative charge density. It is striking found that Tafel equation is just an approximation of novel rate law that can fully describe the i-η relationship and give rise to contrast physical meanings of both Tafel slope and intercept to the traditional. In addition, the established rate law explains why the phenomenological Butler-Volmer theory (out-sphere theory), which focuses on electron transfer, can apply to analysis the OER rates (inner-sphere theory) under a certain circumstances. These findings may thus lead to key insights both for strategies to predict and/or enhance electrocatalytic performances and for understanding OER mechanism at atom/molecular level.