The narrow-band green-emitting phosphor Ba2LiSi7AlN12:Eu2+ was discovered by analyzing a single particle in a powder mixture, which we call the single particle diagnosis approach. Single crystal X-ray diffraction analysis of the particle revealed that Ba2LiSi7AlN12:Eu2+ crystallizes in the Pnnm space group (No. 58) with a = 14.0941 Å, b = 4.8924 Å, c = 8.0645 Å, and Z = 2. The crystal structure is composed of a corner-sharing (Si,Al)N4 corrugated layer and edge-sharing (Si,Al)N4 and LiN4 tetrahedra. Ba(Eu) occupies the one-dimensional channel in a zigzag manner. The luminescence properties were also measured using a single crystalline particle. Ba2LiSi7AlN12:Eu2+ shows a green luminescence peak at approximately 515 nm with a narrow full-width at half-maximum of 61 nm. It shows high quantum efficiency of 79% with 405 nm excitation and a small decrease of luminescence intensity even at 300 °C.