The sticking process of Cl-2 on the Si(100) surfaces has been studied by employing a molecular-beam method. Initial sticking probabilities, S-0, were measured as a function of the incident energy, E-i, and the surface temperature, T-s. The S-0 versus E-i as well as S-0 versus T-s curves were analyzed with a kinetic model, which includes both a direct sticking channel and a precursor-mediated sticking channel. The physisorption lifetimes were measured as a function of T-s to evaluate the depth of the physisorption well and the preexponential factor for the detrapping rate. The potential structure and the intermediate transition state relevant to the Cl-2 sticking on the Si(100) surface were discussed.