The ac electroluminescence of thulium-doped ZnS embedded in boric acid matrix was investigated. The El emission spectra were recorded as a function of voltage, frequency, and temperature. The emission spectra at all temperatures show only strong sharp emission lines which were assigned to transitions within the 4f shell of Tm3+ ion. The emission spectra consisted of five groups of lines centered around 483 nm (1G4 --> H-3(6)), 661.7 nm (1G4 --> H-3(4)), 710.5 nm (3F3 --> H-3(6)), 785 nm (1G4 --> H-3(5)), and 805.5 nm (3F4 --> H-3(6)). The emission intensities (I) of different peaks were recorded as a function of the applied voltage (V). The plot of natural logarithm of I vs. V-1/2 shows a straight line characteristic over many orders of emission intensity which indicates that the direct impact excitation mechanism is a dominant process. The experimental values of the 483 emission line (transition 1G4 --> H-3(6)) intensity vs. the applied voltage were reproduced theoretically on the basis of the direct impact excitation mechanism.