This work investigates the transient current in the subthreshold region of a low-temperature polycrystalline silicon thin-film transistor (LTPS TFT) on a polyimide (PI) substrate. The measurement of this current shows an instability that is not seen in the device on a glass substrate; the instability appears as a current variation under constant voltage and overshoot or undershoot under external stress voltage transitions. To explain this effect for a TFT on a PI substrate, a transient body effect (TBE) is suggested due to a threshold voltage shift caused by the charge distribution inside the substrate. A SPICE model is proposed based on this concept, and it shows excellent agreement with device measurements. The model offers a simulation guideline for LTPS TFTs on PI substrates.