The structure evolution of regioregular poly(3-hexylthiophene) (P3HT) in THF dilute solution and its influence on thin film morphology were studied by a combination of static and dynamic laser light scattering (SLS and DLS) and transmission electron microscopy (TEM). Most P3HT is not directly soluble in THF at room temperature. A thermal treatment at high temperature effectively redisperses P3HT microsized aggregates, and introduces two modes in DLS measurement. The fast mode with < R-h >(f) similar to 4 nm is P3HT unimer coil, while the slow mode with < R-h >(s) similar to 90 nm is P3HT associate. The linear relationship between and q(2) passing through origin indicated both modes are diffusive. DLS showed the percentage of large associates is increasing, and that of the unimer coils is decreasing with temperature. The ratio, < R-g >(s)/< R-h >(s) similar to 0.8, indicates the associates are closely packed spherical in shape. The structure of P3HT in dilute solution greatly influences its morphology on thin films. We found that the memory of chain conformation and association in solution are carried into drop casted film, TEM examination of drop casted thin film morphology clearly indicated that the densely packed spherical associates are elastic with its morphology unchanged during evaporation process because of its visco-elastic nature; while the P3HT unimer coils assemble and crystallize into nanofibrils. The nanofibrils' percentage decreases with temperature because the coexistence equilibrium between P3HT unimer and spherical associates shifts greatly to the associates side. However, this apparent equilibrium of unimer associate is in a metastable state. Over a long time, the system will gradually form larger and more stable aggregates which are in equilibrium with unimers at room temperature. Overall, after a proper thermal treatment in solution, it suppresses aggregates formation and increases electronic and opto-physics properties when drop casting is conducted at room temperature.