Monodisperse polystyrene (PS) films with various thicknesses were spun-coated on silicon wafers with native oxide layer. Surface relaxation behavior in the PS films was studied as a function of thickness by lateral force microscopy (LFM). In the case of a thick PS film, a clear lateral force peak corresponding to surface alpha(a)-relaxation process of segmental motion was observed at a temperature much lower than the bulk glass transition temperature, T-g. As the film became thinner than 3-4 times the radius of gyration of an unperturbed consistent chain, the peak on lateral force vs temperature curve started to broaden out with decreasing thickness and eventually split into two peaks. The appearance temperature of the surface a alpha(a)-relaxation peak was invariant with respect to the film thickness even in such an ultrathin state, meaning that surface T-g was insensitive to the thickness. On the other hand, the ultrathinning-induced relaxation process, called surface beta-relaxation, was strongly dependent on the thickness in terms of relaxation temperature and apparent activation energy. Finally, possible origins of the surface beta-relaxation process were proposed.