Predicting thermoelastic damping is crucial for the design of high Q MEMS devices. In the past, for the thermoelastic damping in microbeam resonators, Zener’s model (1937 Physical Review 52 230-5; 1938 Physical Review 53 90-9) and Lifshitz and Roukes’ model (2000 Physical Review B 61 5600-9) can give a reasonable prediction. However, the derivations of Zener’s model and Lifshitz and Roukes’ model are only suitable for a simple beam with no proof mass. The microbeam with a proof mass is a common element in many MEMS devices. In this paper, a general proof is presented that shows LR’s model is also valid for the TED in the microbeams with a proof mass. The derivation in this paper is based on a general case.