Dynamics of the excited triplet state of pi-conjugated poly(3-n-octylthiophene) in solution was studied by nanosecond laser flash photolysis. The time-resolved absorption spectra for poly(3-n-octylthiophene) in THF showed the triplet-triplet (T-T) absorption band near 800 nm. The fast-and slow-decay components were observed in absorption time profiles of the T-T absorption band, and the ratio of the two components significantly depends on both the polymer concentration and the excitation laser power. With an increase in the excitation laser power, the density of excited triplet state on a polymer chain increases, resulting in the increase of the fast-decay component. The fast-decay component was attributed to the intra-chain T-T annihilation. The rate constant of intra-chain T-T annihilation is three orders of magnitude larger than the diffusion-controlled rate of small molecules in solution as derived from the Debye equation. On the other hand the slow-decay component consisted of mixed-order kinetics with first- and second-order. The first-order part was ascribed to the intrinsic lifetime of the triplet state and the slow second-order part to the inter-chain T-T annihilation, which becomes pronounced when the concentration of polymer is high. The fraction of the fast-decay component in whole-decay components showed significant dependence on the number of triplet state moieties on a polymer chain.