Interfacial electronic structure of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)/Ag substrate was investigated. The interfacial states, derived from the lowest unoccupied molecular orbital of PCBM, are formed around the Fermi level. The vacuum level (VL) was lowered by the positively charged PCBM molecules at the interface, although the VL was raised at C60/Ag interfaces. The x-ray photoelectron spectra revealed that the oxygen atoms of the side chain of PCBM are positively charged at the interface due to the electron transfer to Ag substrate, leading the lowering of the VL. The side chain of PCBM strongly influences the interfacial electronic structure.