An oxynitride phosphor powder has a fluorescence peak wavelength of 587 to 630 nm and a high external quantum efficiency. A method of producing the oxynitride phosphor powder containing Li at 50 to 10,000 ppm includes mixing silicon nitride powder, a substance serving as an aluminum source, a substance serving as a calcium source and a substance serving as an europium source; firing the mixture at 1500 to 2000° C. in an inert gas atmosphere or a reducing gas atmosphere to obtain a fired oxynitride phosphor composed mainly of Ca-containing α-SiAlON, as an intermediate; and heat treating the fired oxynitride phosphor at a temperature of 1450° C. to less than the firing temperature, in an inert gas atmosphere or in a reducing gas atmosphere in the presence of Li.