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Porous polyimide films were prepared in two steps. The first step was the preparation of poly(urethane-imide) films by a reaction between a phenol-terminated polyurethane prepolymer and poly(amide acid) obtained from pyromellitic dianhydride (PMDA) and oxydianiline (ODA) followed by thermal treatment at 200 • C. The second step was the pyrolysis of the poly(urethaneimide) films. Upon thermal treatment of the poly(urethane-imide) films at 300 to 400 • C, the thermally less stable urethane domains decomposed, leaving porous polyimide films. The presence of pores was confirmed from the scanning electron microscope (SEM) picture of the surface and the cross section of the films. The size distribution of the pores was narrow. With increasing urethane content, the apparent density decreased but the size of the pore did not increase so much. Instead, the number of pores increased. Tensile measurements revealed that, with increasing urethane component the tensile modulus did not vary very much, but the elongation and thus the tensile strength decreased. Viscoelastic analyses showed that all the porous polyimide films had a high glass transition temperature, above 400 • C, and storage moduli were maintained up to 400 • C.