Zinc sulphide (ZnS) and copper-doped zinc sulphide nanocrystallites (ZnS:Cu) of average size 4 and 3 nm, respectively, have been synthesized by chemical precipitation method. Structural and morphological studies using X-ray and high resolution transmission electron microscopy (HRTEM) measurements have confirmed hexagonal structure for the samples. Using impedance spectroscopy, the effect of grain interior and grain boundary regions on the electrical conductivity have been studied at various temperatures. In the high temperature region, the grain boundary contribution to conduction is found to be larger than that of the grain interior region. Further, the activation energies of charge carriers in both the grain interior and grain boundary regions have been determined. The conduction mechanism of copper-doped zinc sulphide nanocrystallites have been studied at various temperatures and the results are reported.