Potassium niobate (KNbO3) microcubes with cubic, orthorhombie and tetragonal phases were hydrothermally synthesized.The photoreactivity of the as-prepared KNbO3 samples was evaluated toward hydrogen evolution from an aqueous methanol solution under UV and shown to be phase-dependent, following the order cubic > orthorhombic > tetragonal.The origin of the underlying mechanism was unraveled by first-principles density functional calculations.The best photoeatalytic performance of cubic KNbO3 is ascribed to its highest symmetry in the bulk structure and associated electronic structure.In addition, surface electronic structure plays a key role leading to the photoreactivity discrepancy between orthorhombic and tetragonal KNbO3.