In order to investigate the energy transfer process and the mechanism of the sensitization effect of copper and sodium tons on Er luminescence in ZnS: Cu, Er, Na phosphors, emission spectra, time-resolved emission spectra, transient behavior and dependence of intensities of green copper (G-Cu) and Er luminescence have been measured, It is confirmed that the excitation of Er ions is caused by the nonradiative energy transfer from the G-Cu centers to the Er centers. The experimental results show that (i) the copper-ions, which form the G-Cu centers, act as the sensitizor of Er luminescence, (ii) but the sodium ions do not act as the sensitizor, but act as the charge compensators for the trivalent erbium ions.