A relatively narrow molecular weight dispersed regioregular poly(3-hexylthiophene) (rr-P3HT) fraction was obtained through the removal of low molecular weight fraction by Soxhlet extraction and the high molecular weight fraction by ultracentrifugation. Combined static (SLS) dynamic (DLS) light scattering and TEM measurements were conducted to study its conformation variation with temperature. An upper critical solution temperature (UCST) type of phase diagram with a loose aggregate to unimer transition process is observed in the heating process. The dynamic equilibrium between unimer, with < R(h)> similar to 4 nm, and loose aggregate, with < R(h)> similar to SS nm, shifts to the unimer side with temperature because the loose aggregate, which consisted of bundles of unimer chains, are destroyed and dissembled by thermal energy. TEM observation was used to verify our LS results. It is found that the presence of loose aggregate in solution is crucial for the formation of interconnected and long nanofibrils in film morphology as loose aggregate embraces interchain interaction between conjugated polymer chains. Independent nanofibrils come from the crystallization of unimers during solvent evaporation process. Therefore, the percentage of interconnected nanofibrils decreases with drop-casting temperature. In the cooling process, this kinetic of self-assemble from unimers into loose aggregates in the THF solution is very slow. Taking advantage of this slow kinetic, we studied the time dependence of the structures formation at 10 degrees C. The unimers to loose aggregates to spherical associates to precipitates transition processes are monitored. This kinetic mapping of the unimer to loose aggregate equilibrium to nonequilibrium large scale associates is essential to optimize the nanofibrils' percentage and structure before the drop-casting process.