A green phosphor, La0.4Ca13.3Eu0.3Mg2Si8O31.6+N0.4- (LaCMSN:Eu2+), was prepared by a solid-state reaction and an efficient green emission was observed at 506 nm under near-ultraviolet (NUV) excitation. The structural and optical properties of LaCMSN:Eu2+ phosphors as well as their thermal quenching were investigated. The partial substitution of La3+ and N3- in Ca13.7Eu0.3Mg2Si8O32 led to a considerable enhancement in the peak emission intensity by as much as 194%. This demonstrates not only that the total number of Eu2+ activators increased, but also that the probability of a nonradiative transition between Eu2+ and Eu3+ could be reduced as the increase in concentration of the former is at the expense of the later. The white light-emitting diode (LED) was fabricated using phosphor with a NUV LED chip. The LED showed warm white light with an excellent color rendering index of 91. The LaCMSN:Eu2+ is thus a potential green-emitting phosphor for white LEDs.