Characteristics of transient drain current overshoot in poly-Si TFTs are measured, and an equivalent thermal circuit model is proposed based on the experimental results. By changing the terminals on which a step voltage is applied, two main mechanisms causing the transient current, i.e., the electron trapping effect and the self-heating effect, can be separately evaluated. Using this new technique, we discuss the heat conduction mechanisms in TFTs responsible for describing the transient current overshoot component induced by the self-heating effect.