In this study, GaP QDs were synthesized by a facile hot injection method. The optical properties such as absorption and PL emission were investigated and finely controlled by varying synthetic parameters such as surfactant loading, precursor composition, and fatty acid. The obtained GaP QDs were applied as a color conversion material on LED based devices. Display omitted Group III–Ⅴ quantum dots (QDs) are preferred over Group Ⅱ–Ⅵ materials because of their relatively mild toxicity. In this study, a facile synthetic method to obtain gallium phosphide (GaP) QDs with color conversion performance is reported. Colloidal GaP QDs were produced via a hot-injection method using an optimized combination of precursors. The products showed controllable emissions from 400 to 520 nm, depending on the band gap. High photoluminescence (PL) quantum yields of 35–40% with a full width at half maximum (FWHM) of 75 nm were achieved in the green emission region. In addition, the green-emission GaP QDs were applied as a color-conversion material for optical devices with UV and blue LED chips. An average color conversion efficiency of 15% was achieved. This study demonstrates the possibility of using GaP QDs as a competitive color-conversion material.