Charge transfer behavior of Poly(3‐hexylthiophene) (P3HT) and 6,6‐phenyl‐C61‐butyric acid methyl eser (PCBM) in solutions and in films were examined by photoluminescence (PL) spectroscopy. PL study in solutions indicated that separation distance between P3HT and PCBM affected charge transfer efficiency more seriously than the interface area issue between P3HT and PCBM. P3HT/PCBM film showed very effective photo‐induced charge transfer before post‐thermal annealing on the bi‐layer P3HT/PCBM film. Charge transfer efficiency was gradually diminished by the annealing‐induced phase separation between P3HT and PCBM as revealed by increasing PL emission intensity of P3HT. Charge transfer behavior between P3HT and PCBM was examined. Photoluminescence study indicated separation distance between P3HT and PCBM affected charge transfer more seriously than interface area issue between P3HT and PCBM. P3HT/PCBM film showed effective photo‐induced charge transfer before post‐thermal treatment and diminished by annealing‐induced phase separation between P3HT and PCBM as revealed by increasing PL emission intensity of P3HT.