We report on the self-assembly of poly(3-dodecylthiophene) (P3DDT) into nanowhiskers for the first time via addition of the marginal solvent anisole into its well-dissolved solution. By controlling the solvent composition and aging time, we observed a morphology evolution from nanowhiskers to two-dimensional nanoribbons and foliated aggregates, which was ascribed to diverse driving forces for self-assembly in the process of crystallization. UV-vis absorption spectroscopy and dynamic lighting scattering (DLS) measurements were employed to in situ monitor crystallization kinetics of P3DDT induced by mixed solvents. It has been shown that conformational transition serves as a critical factor for P3AT to perform pi-pi stacking to form nanowhiskers. From a thermodynamic point of view, P3AT dispersion dissolved in mixed solvents is actually not a thermodynamic equilibrium system, but a multicomponent and multiphase case whose phase composition and properties evolve with time. The understanding in morphology transition mechanisms and crystallization kinetics of P3DDT can provide guidelines for optimization of processing parameters and enhance performance of photovoltaic devices.