A new phase of Gd 5 Si 3 O 12 N has been identified in the Gd-Si-O-N quaternary system. With Rietveld refinement technique, the crystal structure has been refined. Gd 5 Si 3 O 12 N belongs to the hexagonal system with space group of P 6 3/m (No. 176). The lattice parameters are a = b = 9.6838(1) Å, c = 7.2119(1) Å, α = β = 90 o and γ = 120 o . Gd atoms occupy two types of crystallographic sites in the structure. With Eu 2+ doped, two types of photoluminecence centers are formed. Gd 5 Si 3 O 12 N:Eu 2+ is a novel kind of green emitting phosphor. An abnormal blueshift of the emission band has been observed. Concentration quenching occurs at Eu 2+ concentration of 2 mol%. With the analyses of the structure and the photoluminescence, the physical mechanisms for the emission blueshift and for the concentration quenching were investigated and discussed.