Red phosphor is indispensable to achieve warm white light in the white light diode (WLED) application. However, the current red phosphors suffer from high cost and harsh synthesis conditions. In this study, an oxide-based rare-earth-free red-emitting phosphor Li3Mg2NbO6:Mn4+ (LMN:Mn4+) has been successfully synthesized by a simple solid-state reaction method. The relationship between crystal structure and luminescence was investigated in detail. The site occupancy of the doping Mn4+ ion in the LMN host has been discussed from the point of bond valence sum. How the coordination environment of doping Mn4+ affects the energy level of doping Mn4+ ion has been illustrated via the Tanabe-Sugano energy-level diagram. Moreover, warm white light has been obtained using LMN:Mn4+ as compensator to the YAG:Ce3+.