One of critical issues of TFT for large scale integration is temperature rise of TFT due to self-heating. In this paper, we report temperature rise of poly-Si TFT during operation. The temperature was evaluated by determining the thermal resistance from the temperature-dependent negative-drain conductance. TFTs are fabricated using a laterally-grown poly-Si film. By aligning TFT channel direction with the grain growth direction, effects of grain boundary on carrier transport becomes less significant so that direct evaluation of self-heating from drain characteristic becomes possible. SOI MOSFET is also investigated. Results indicate that the thermal resistance of TFT is 40 times as large as that of SOI MOSFET. As a consequence, temperature rise of TFT reaches to 150 K even under normal operation condition. Heat dissipation path is also investigated by determining the thermal resistance of TFTs having various dimensions. Effect of stripe channel on TFT performance and temperature rise is also discussed. Results clearly indicate that design of thermal path through the gate becomes of importance for TFTs.