Semiconductor ZnS quantum dots (QDs) co-doped with Eu³⁺ and Ce³⁺ were synthesized by the chemical method. The optical properties of the samples were characterized by absorption spectroscopy, photoluminescence (PL) spectroscopy and PL-decay lifetime. X-ray diffraction revealed that the QDs had a zincblende structure and a particle size of about 3 nm. The presence of Eu³⁺ and Ce³⁺ ions in the samples were proved by X-ray photoelectron spectroscopy. The properties of ligand field and the intensity parameters of Eu³⁺ doped ZnS QDs were analyzed by Judd-Ofelt theory. For Ce³⁺ and Eu³⁺ ions co-doped in ZnS QDs, the luminescence intensity and lifetime of the 5d₁ (Ce³⁺) level decrease while the emission intensity of Eu³⁺ ion increases with the increasing Eu³⁺concentration. The reduced lifetime and the luminescence quenching of the 5d₁ (Ce³⁺) level are due to the energy transfer from Ce³⁺ to Eu³⁺ ion. The efficiency of the energy transfer process from Ce³⁺ ions to Eu³⁺ ions and the nature of this interaction mechanism were determined by applying Reisfeld and Inokuti-Hirayama models.