Here, we report a novel strategy to prepare fluorescent semiconductor quantum dots (QDs) of core–shell type with CdSe–CdS QDs as a model system. Our synthesis was carried out in liquid paraffin, which is a natural, nontoxic, and cheap solvent. We applied a single injection of precursor for the shell growth at low temperature and gradual heating of the reaction mixture after that. By this manner, the Ostwald ripening of the cores was reduced, homogenous nucleation of the shell material was avoided, and highly monodisperse in size core–shell QDs were prepared. Our synthesis method allows working on open air; it is relatively fast and allows fine control over the shell growth process. It leads to the formation of core–shell CdSe–CdS QDs with fluorescence quantum yield as high as 65%. We described the optical properties of core–shell QDs by the model of attenuated quantum confinement.