Bulk heterojunction organic solar cells based on small molecules are often fabricated by the co-deposition of donor and accepter materials on substrates placed horizontally. An intimate blend of donor and acceptor molecules is the common product in as-prepared samples. Using metal phthalocyanine as the donor and fullerene as the acceptor, we tilt the substrate to deposit the active layer. Both short circuit current density and open circuit voltage increase obviously in bulk heterojunction organic solar cells fabricated by oblique angle deposition. Analysis of the active layer reveals that phase segregation occurs in obliquely deposited bulk heterojunction layers. The mechanism of phase segregation is discussed. A change in the stacking style of fullerene molecules on metal phthalocyanine grains is proposed on the tilted substrate. This result provides a simple method to form phase segregation in bulk heterojunction layers.