The excited states of pi-conjugated poly(3-octylthiophene) were studied by laser flash photolysis technique. The nanosecond time-resolved absorption spectra for poly(3-octylthiophene) showed the triplet-triplet (T-T) absorption band (lambda(max) 800 nm) in the near-IR region. The intersystem crossing process was investigated by picosecond time-resolved absorption spectroscopy. The absorption-time profile obtained by picosecond laser flash photolysis of poly(3-octylthiophene) using a streak camera showed an initial rising component just after the photolysis and a slow growing component within a few nanoseconds. They were assigned to excited singlet and triplet states, respectively. The absorption-time profiles were analyzed by curve-fitting considering the decay of the excited singlet state and the growth of the excited triplet state. The intersystem crossing rate constant from the excited singlet state to the excited triplet state was determined to be 1.54 x 10(9) s(-1) by the curve-fitting. The S-l-S-n and T-T absorption bands were depicted separately using the fitting parameters.