Thin films of CdS on glass substrates have been prepared by a thermal evaporation technique using a modified evaporation source under different preparation conditions. The structural features have been investigated using XRD. The current density-voltage characteristics have been investigated at room temperature with different thicknesses. The results showed ohmic conduction in the lower voltage range and space-charge limited conduction controlled by a single-discrete trapping level for a higher voltage section. Analyses of the results yielded important dependence of film thickness on both the thermally-generated electron concentration and the trapping factor. The optical parameters have been calculated. The dependence of the refractive index, n, and extinction coefficient, k, on the wavelength for a prepared sample is also reported.