An experimental study of energy transfer collisions,Cs(6P)+Cs(6S)+He→Cs(5D)+Cs(6S)+He,at thermal energies,was carried out in a cell. Atoms were excited to the 6P_3/2 state using a single-mode diode laser. The Cs(6P)atom density and spatial distribution were mapped by monitoring the absorption of a counter-propagating laser beam. The buffer gas enhanced the energy transfer between Cs(6P)and Cs(6S),which was based on the measurement of the fluorescence from the 7D state populated by the Cs(6P)/Cs(5D)collisions. Since the 5D→6P fluorescence(3.0～3.6μm)can not be detected with this experiment,the rate coefficient has been measured relative to the known rate coefficient of the collision(i.e.,6P+5D→6S+7D). The three-body collision rate coefficient [k3 =(2.2±1.1)×10^－28cm⁶s^－1]is obtained.