Single crystals of K_0.14Na_0.86NbO₃ (KNN) and Mn-substituted KNN (Mn-KNN, K_0.14Na_0.86Mn_0.005Nb_0.995O_y) were grown by a flux method, and the effects of Mn substitution on the leakage current, polarization and dielectric properties were investigated along [100]_cubic. As-grown and annealed (850°C in air) crystals did not show an apparent polarization hysteresis loop due to its large leakage current density (∼10⁻³ A/cm²). The annealing at 1100°C in air led to a marked decrease in leakage current density of the order of ∼10⁻⁶ A/cm² for KNN crystals. Mn-KNN crystals annealed at 1100°C in air exhibited a low leakage current density of ∼10⁻⁸ A/cm². Electron spin resonance measurements and defect chemistry analysis show that the average valence increase of Mn by oxidation treatment (the annealing in air) absorbs electron hole, the carrier of the leakage current, which is suggested to be the origin of the low leakage current observed for Mn-KNN crystals.