A simple process to fabricate porous polyimide membranes (PPMs) and effect of warm-up temperature rates on morphologies of the PPMs were reported in this paper. The polymer solution layer consisting of the corresponding polyamic acid (PAA), solvent and the pore forming agents with high boiling temperature or high decomposition temperature was first treated under a lower temperature (about 150°C). The received solid membrane was further imidized by a higher temperature (about 270°C) and the pore forming agents were removed from the membrane at a temperature above their boiling temperature or decomposition temperature at last. Then a PPM was obtained. As the temperature was elevated gradually, the porosity of the received PAA membrane was lower than that of the PPM treated by a faster warm-up temperature rate.