An unusual increase in Tg of polyimide composite film consisting of semi-alicyclic polyimide (PI-1) and thermally treated talc (b') was observed by TMA measurement, where the Tg jumped as high as +50 K from the original one with 45 wt% of the baked talc. The degrees of (i) increasing viscosity of the solution with PI-1 precursor and the talc, (ii) increasing Tg of the composite film, and (iii) increasing tensile modulus of the composite film were all corresponded to the amount of thermal weight loss from the original talc. According to XRD and TEM observations, the morphological change of the stacking layer structure inherently talc owns was not occurred while the talc was subjected to bake at 700 °C in nitrogen. Since TG-MS elucidated that the emerged components during the thermal treatment was assignable to carbon dioxide, the original talc included some carbonate impurities. The volume of that contaminant involved in talc thus supposed to be correlation to the film thermal property.