A plastic thermistor comprising a polyamide composition which comprises 100 wt. parts of a polyamide and 5.3 to 30 wt. parts of zinc iodide, or a polyamide composition which comprises a polyamide, an iodine-containing compound, and a metal oxide such as zinc oxide. Ion carrier properties of a metal iodide greatly increases temperature dependence of impedance, and the metal oxide such as zinc iodide functions as a receptor for iodide ions and prevents the formation of a metal iodide on the surfaces of metal electrodes. Furthermore, a linked cycle can be established that zinc oxide forms zinc iodide and then formed zinc iodide functions to increase the stability of half-wave current passage. Accordingly, the thermal stability of the plastic thermistor is improved for a long time, and the heat resistance stability of temperature sensors or thermosensitive heaters comprising the plastic thermistor is improved greatly.