Photo-physical and nonlinear-optical properties of a new amphiphilic conjugated polymer: hydroxylated pyridyl para-phenylene (Py-PhPPP) both in CH2Cl2 solution and in thin films have been investigated. By using Z-scan technique with nanosecond laser pulses of wavelengths ranging from 430 to 600 nm, the large nonlinear absorption and refraction have been determined in terms of the effective third-order, nonlinear-optical susceptibilities. These Z-scans reveal that the nonlinear absorption alters from reverse saturable absorption to saturable absorption at wavelength of ~ 540 nm. Similarly, alteration from self-defocusing to self-focusing manifests itself at the same wavelength. The optical limiting performance of Py-PhPPP in solution is superior to toluene solution of [60]fullerene (C60) at 532 nm. Both UV-visible absorption spectra and photoluminescence (PL) spectra show concentration dependence. The PL spectra also depend on excitation wavelengths. These evidences suggest that the aggregate formation should play an important role in the nonlinear-optical properties of the new polymer. We attribute the reverse saturable absorption in the region of blue and green wavelengths mainly to intra-chain, triplet-triplet absorption while the absorption bleaching at longer wavelengths is due to saturation in the absorption band induced by the aggregates.