(Ce0.67Tb0.33-x)MnxMgAl11O19 phosphor was synthesized by a sol-gel process. The excitation spectra were characteristic of the Ce3+ cation, while the emission spectra consisted of lines from Tb3+ and Mn2+. Mn2+ ion gave green fluorescence band and concentration quenching occurred when x > 0.09. By analyzing the crystal structure of the phosphor, a reasonable explanation is given for the optical properties of the phosphor.