Nanoparticles of regioregular poly(3-hexylthiophene) (P3HT) with different molecular weights were prepared by reprecipitation method. Particle sizes of the nanoparticles were estimated approximately 30?nm by dinamic light scattering measurement and field effect scanning electron microscope observation. The nanoparticle dispersions exhibited thermochromic behaviors. Absorption bands corresponding to the P3HT crystalline phase were observed at 25??C. As temperature increased, the bands ascribed to the crystalline phase were decreased and those due to the amorphous phase increased. The spectra of the nanoparticle dispersions at 95??C were almost the same as those in the solution state. By comparing to areas of absorption spectra corresponding to the crystalline and amorphous phase, it was found that the crystallinity of the nanoparticles was strongly dependent on the molecular weight.