First-principles calculations using density functional theory based on norm-conserving pseudopotentials have been performed to investigate the Cs adsorption on the Si(001) surface for 0.5 and 1 ML coverages. We found that the saturation coverage corresponds to 1 ML adsorption with two Cs atoms occupying the double layer model sites. While the energy bands spectra of the 0.5 ML covered surface is of metallic nature, we found that 1 ML Cs adlayer leads to a semiconducting surface. The results for the electronic behavior and surface work function suggest that adsorption of Cs takes place via polarized covalent bonding. The nature of the Cs-Si bond shows a hybrid character of s-p type implying a limited charge transfer from the adsorbate to the substrate.