Concentration dependence of Mn2+ luminescence in NaCaPO4/Mn2+ is investigated by structural analyses and optical and laser excitation spectroscopies in the temperature range 19-300 K. NaCaPO4/Mn2+ forms solid solution over the Mn2+ concentration range 1.0-22 mol %. We observe the red shift and unusual band narrowing of Mn2+ emission by increasing Mn2+ concentration in NaCaPO4. The lifetime of Mn2+ emission lengthens unexpectedly for higher Mn2+ concentration. The results are discussed in relation with crystal structure, photon reabsorption, exchange interaction, and energy transfer and energy migration in NaCaPO4/Mn2+ .