Polyimide (PI)-based composite films were prepared by efficient solution casting and following thermal treatment of poly(amic acid) (PAA) precursor, which contained different acid-treated multiwalled carbon nanotube (aMWCNT) contents of 0.1-10.0 wt%. The microstructures, thermal and electrical properties of the composite films were characterized as a function of the aMWCNT content. SEM images of the composite films revealed that aMWCNTs were well dispersed in the PI matrix. Thermogravimetric data demonstrated that the composite films were thermally stable up to ~450 °C. The electrical resistivity of the composite films decreased considerably from ~1011 Ω cm to ~102 Ω cm with the increment of the aMWCNT content, dominantly at a certain percolation threshold between 1.0 and 3.0 wt% aMWCNT. The composite films with above 5.0 wt% aMWCNT contents showed excellent electric heating performance. For example, the composite film with 10.0 wt% aMWCNT exhibited low temperature growth/decay time constant of < 1 s, stable maximum temperatures of 30-310 °C, and high electric power efficiency of ~4.07 mW/°C under applied voltages of 10-70 V. The overall results manifested that PI/aMWCNT composite films could be used as excellent performance electric heating materials with high thermal stability.