The considerable performance enhancement of P3HT-based solar cell after thermal annealing can be attributed to the thermodynamically driven partial crystallization and phase segregation of each component. In the present work, thermal annealing was executed by delivering P3HT∶PCBM blend films onto the preheated susceptor in a PECVD chamber filled with high purity nitrogen gas. The pressure of inner chamber could be set steadily and precisely in the range of 1 to 1 850 mTorr at 150 ℃. It was found that the phase segregation scale of two components was tuned to a certain extent by varying the annealing pressure, whereas the polymer crystallinity was slightly affected. According to the pressure settings, polymer solar cells (PSCs) were fabricated in the following structure: ITO/PEDOT∶PSS/P3HT∶PCBM/Al. All of the device parameters exhibited the similar trend—an initial increase followed by a decrease, and reached a peak at 1 500 mTorr with successive increase in annealing setting pressure. PSC annealed under 1 500 mTorr shows overall high performances with the power conversion efficiency up to 3.56%. The UV-Vis absorption spectra of annealed blend films shows that the vibronic absorption peaks (shoulders) at 510, 550 and 600 nm became more pronounced under higher setting pressure, which is attributed to better crystalline P3HT with increased π—π stacking of polymer molecules. The AFM results further suggest that high annealing pressure (>1 000 mTorr) promoted the domain formations of P3HT or PCBM;moreover, a moderate phase segregation, as a result of an appropriate annealing pressure (1 500 mTorr), facilitates polymer crystallization which ensures the high charge (hole) mobility and consequently increased short-circuit current and fill factor.