In this work, we report the efficiency enhancement of a GaAs solar cell by using a luminescent down-shifting (LDS) layer consisting of (CdSe)ZnS quantum dots (QDs). The calculated conversion efficiency shows strong dependence on the luminescence quantum efficiency (LQE) and concentration of QDs in the LDS layer as well as the difference in external quantum efficiency (EQE) of the solar cell in the absorption and emission regions of the QD. Although the irradiance of the modified spectrum by the LDS layer was reduced with increases in QD concentration, the emission range of QDs in the spectrum was intensified. So the optimum parameters need to be calculated carefully. When we measured the GaAs solar cell with the LDS layer, it showed increased current density and consequently improved efficiency. The highest performance difference between the GaAs solar cell efficiencies with and without the LDS layer was over 2.8%, although a crystalline silicon solar cell below the LDS layer showed negative change in efficiency owing to its small EQE difference.