Hole-burning spectroscopy and ionization threshold measurement were combined with fluorescence excitation (FE) and resonant two-photon ionization (R2PI) spectroscopies to probe conformation dependence of photoionization for jet-cooled 1,3-diphenylpropane (DPP). The S-1<--S-0 excitation energy dependence of excimer fluorescence and that of one-color R2PI indicate formation of an intramolecular excimer and an efficient photoionization of the excimer that occur at higher energies. The energy threshold for the enhanced photoionization, which concurs with that for the excimer formation, is considerably lower for the trans-gauche (tg) conformer than for the trans-trans (tt) or the gauche-gauche (gg) conformer. These results demonstrate the occurrence of conformation selective excimer-mediated photoionization in DPP. The important precursor role of the singlet excimer in the photoionization is attributed to the conformational similarity between the intramolecular singlet excimer and the most stable ground-state cation of DPP, which leads to a very favorable Franck-Condon factor for the photoionization of the excimer. Even for excitation energies far below the energy threshold for excimer formation, the efficiency of one-color R2PI is substantially greater for tg than for tt or gg for photoionization with a high intensity laser. Comparison of the ionization threshold measured with high photon flux with that obtained with low photon flux suggests that the greater ionization efficiency of the tg conformer at high photon flux may be related to direct photoionization of the S-1 tg to the ionic state of DPP with T-shaped arrangement of the phenyl moieties.