The Brillouin scattering was used to investigate the effect of crosslinking on the dynamic mechanical properties and the relaxations of polystyrene. It was found that there is a new relaxation in the cross-linked polystyrene besides the high temperature (T>Tg) amorphous relaxation. The effect of crosslinking on this relaxation was examined. The activation energy of this relaxation was determined to be about 12 kcal/mol. The molecular origins of this relaxation were proposed.