The nominal composition of AlO-Ce:YMgAlSiO (A-Ce:YMASG) ceramic phosphors was fabricated by the vacuum sintering technique. The introduction of AlO as a second phase partially enters the crystal lattice, which was confirmed by the composition changing of the samples through x-ray diffraction measurement. An impurity phase of YMgSiO was observed in Ce:YMASG and disappeared with the introduction of AlO at the concentration of 10 wt. %. When the content of AlO increased to 30 and 50 wt. %, the second phase of AlO was measured with actual weight ratios of 7.72 and 20.55 wt. %, respectively. The third phase of MgAlO was found with the further addition of AlOat 70 wt. %; the weight ratios of Ce:YMASG, AlO, and MgAlO were 68.756, 18.457, and 12.787 wt. %, respectively. The luminescent characters of the samples were measured by the photoluminescence spectra and electroluminescent spectra. With the increase of AlO from 0 to 30 wt. %, the emission wavelength of Ce plummeted from 610 to 552 nm, and the luminous efficacy of the samples increased from 35 to 65 lm/W.