Na+, Mn4+ co-doped Sr4-x/2NaxAl14-0.01x4/3Mn0.01O25 phosphors were synthesized by solid-state reaction at 1400 degrees C in air. The excitation spectra of synthetic samples showed that the phosphors were effectively excited by both near-ultraviolet light and the commercially available blue light of LED chips. The optimized concentration of Na+ in phosphor was 5 mol% and the strongest emission intensity of the phosphor increased by 160% compared with its counterpart without Na+ co-doping. The emission spectra of Sr4-0.05/2Na0.05Al14-0.001x4/3Mn0.01O25 showed two emission peaks (654 nm and 664 nm) in the deep-red region, corresponding to CIE (Commission International de I' Eclairage) chromaticity coordinates (0.72, 0.28). The results suggested that the Na+, Mn4+ co-doped phosphor is a good candidate as a red phosphor in warm white light emitting diodes due to its excellent luminescence and easy accessibility of raw materials. (C) 2014 Elsevier B.V. All rights reserved.