Preparation of CdS nanoparticles, devices fabrication, and electroluminescence properties at room temperature of CdS nanoparticles on silicon substrates are reported. A spectral shift of 86-meV of free exciton transition was observed that was due to the passivation of p-hydroxyl thiophenol molecules around nanoparticles. Controlled process conditions such as heat treatment and/or with oxygen-rich environment are experimented and found to have significant influences on emission spectra. Radiative recombination corresponding to oxygen-impurity level, 273 meV below bandgap energy, presents in samples prepared in oxygen-rich environment. In addition to such mechanism, coalescence of nanoparticles into bulk form also exists and contributes to enhanced luminescence.