Normally, it is difficult to directly measure the bandgaps of perovskite based on methylammonium (MA) or formamidinium (FA) at high temperatures due to material decomposition. We prevent the decomposition by keeping the synthesized perovskite films (MAPbI₃and MAPbI₃) in organic iodide vapors, then measure the in-situ resistance of the films at varied temperatures, and further evaluate the bandgaps of these two materials. The evaluated bandgaps are consistent with the results from ultraviolet-visible (UV-vis) absorption spectrum. The bandgap of MAPbI₃decreases with temperature above 95 °C, whereas that of FAPbI3 first increases with temperature from 95 °C to 107 °C and then decreases with temperature above 107 °C.