According to thermal expansion principle,a novel real-time method is proposed in this paper for determining the glass transition temperature (Tg) of thin polymer films based on the attenuated total reflection technique. The method is that the changing values of the refractive index and the thickness of the polymer film can be determined with the ascending temperature using the real-time system. Experimental results definitely show that curves of temperature dependence of the thermal expansion coefficient (β) and the refractive index (dn/dT) change discontinuously and exist a same intersection point. The temperature of intersection point is the polymericTg. With host-guest DR1/PMMA,the measurement procedures and experimental results are given. The repeatability and reliability of measuring Tg of polymer by this method are examined.