The effect of a thermal annealing treatment on the performance of bulk heterojunction photovoltaic cells based on poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) and fullerene (C 60 ) composites is investigated. Upon thermal annealing at 120 ∘ C, short-circuit current (Isc) and power conversion efficiency (η) are more than tripled, while a sharp rise by eight times in Isc and η is found for the device annealed at 200 ∘ C. It is concluded that the improved phase separation between MEH-PPV and C 60 leads to the enhancement of Isc and η at 120 ∘ C, while thermodynamic molecule arrangement at the higher temperature of ∼200 ∘ C induces a significant increase in all photovoltaic parameters of composite devices except the open-circuit voltage (Voc).