Phenyl-C71-butyric acid methyl ester （［70］PCBM） clusters are investigated by using the B3LYP method with 6-31G（d） basis set. The optimized results indicate that the addition of PCBM into the ［6，6］-junction produces a closed methanofullerene which is thermodynamically stable product; and the addition into the ［5，6］-junction results in an enlarged fullerene （an open fulleroid） which is a kinetically controlled product. The first adiabatic electron affinity for ［70］PCBM is similar to that for C70. The energy gaps of ［70］PCBM are reduced compared with those of C70. PCBM derivatives and show increased level of the lowest unoccupied molecular orbital of fullerenes. From the natural charge populations， it is found that adding PCBM unit onto the C70 cages does not change the charge populations remarkably; attaching a PCBM has no effect on the electronic structures of C70. The results of theoretical calculation suggest that PCBM is not involved in the process of photoelectric conversion but it plays a key role in adjusting the level of HOMO-LUMO for increasing photoelectric conversion efficiencies.