Transient absorption spectra of an aromatic polyimide, PI(PMDA/ODA), and a polyimide with an alicyclic diamine, PI(PMDA/DCHM), were determined in order to investigate the main active species in the photoconductivity of the polyimide films by picosecond pulse radiolysis method. A model compound, N,N’-di-n-butylpyromellitic diimide (DBPMDI), was utilized to investigate the active species of pyromellitic imide moieties in solutions of dichloromethane (DCM) and tetrahydrofuran (THF). In the aromatic polyimide film, the radical anions of the electron-accepting pyromellitic imide moieties at 660 and 720 nm and the radical. cation of diphenyl ether moieties at 550 nm were determined by electron transfer from the aromatic ring to imide moieties in the polymer backbone. In the polyimide film with the alicyclic diamine, by utilizing the model system, only the radical anions of pyromellitic imide moieties could be assumed at 660 and 720 nm as the active species in the photoconductivity of the polyimide film. These active species, such as radical cations or radical anions, are expected to be the charge carriers of holes or electrons under the applied electric field, and the transport of these charge carriers would result in the photoconductivity of the polyimide films.