The photoluminescence (PL) properties of europium-doped CaMgSiO4 phosphors were systematically studied. Eu3+-doped CaMgSiO4 exhibited an intense red emission from the D-5(0) to F-7(J) manifold transitions of Eu3+, whereas the Eu2+-doped phosphors showed an extremely weak emission peaking at 475 nm due to the 4f(6)5d(1) to 4f(7) transition of Eu2+. The PL studies indicated that a complete reduction in Eu3+ to Eu2+ is not possible in CaMgSiO4. The vacuum ultraviolet spectrum of the undoped CaMgSiO4 showed that the host absorption energy was at ca. 150 nm. Based on the bandgap energy, the location of the energy levels of Eu2+ and the Fermi level relative to the band structure of CaMgSiO4 was deduced to discuss the valence stability of Eu and the luminescence behavior of CaMgSiO4:Eu2+. In addition, the electronic structure of CaMgSiO4 was investigated by using the density functional theory with the local-density approximation.