This paper reports the influence of annealing treatment on the performance of organic solar cells. The structure of ITO/PEDOT:PSS/P3HT:PCBM/LiF/Al bulk heterojunction solar cells were fabricated and annealed with three different temperature of 50°C, 100°C and 150deg;C respectively. IV characteristic of the device found that the annealing treatment might optimize the solar cell performance. For optimum device performance, maximum temperature at 100°C give the short-circuit current density (Jsc) of 0.55 mA/cm2 and energy conversion efficiency (℘) ca. 2%. The experimental and the physics behind the improvement will be discussed in this paper.