Single-phased orthorhombic nitridosilicate CaSiN₂ powders were prepared at 1550 °C in a N₂/H₂ (6%) atmosphere by solid-state reaction using Ca₃N₂ and Si₃N₄ as the starting materials. The crystal structure refinements were carried out by the Rietveld method based on the X-ray and neutron powder diffraction data collected at 297 K, respectively. The CaSiN₂ phase crystallizes in an orthorhombic unit cell with the space group Pbca (No. 61) and cell parameters a = 5.1334(3) Å, b = 10.3090(5) Å, c = 14.5756(5) Å, Z = 16 (XRD data). Instead of the ideal formula CaSiN₂, small amounts of oxygen could be detected in the samples. The O/N composition of orthorhombic CaSiN_2−2δ/3O_δ was analyzed by EDX and Rietveld refinement on the neutron powder diffraction data. SAED and HREM characterizations of the crystallites were conducted before the EDX analysis in order to select proper crystallites and exclude other minor second phases of (oxo)nitridosilicates such as cubic CaSiN₂ and monoclinic CaSi₂O₂N₂. The luminescence measurements indicate that Ce³⁺/Li⁺ co-doped CaSiN_2−2δ/3O_δ emits a typical shouldered yellow band peaking at 530 nm (FWHM 120 nm), which originates from the 5d → 4f transition of Ce³⁺. The emission band shifts to longer wavelengths with an increment of Ce³⁺ concentration. The strong excitation band appears in the range of 350–470 nm, which is very favorable for the applications in near-UV and blue phosphor-converted light-emitting diodes (pc-LEDs).