We demonstrate the power conversion efficiency of bulk heterojunction organic solar cells can be enhanced by introducing Ag nanoparticles into organic exciton blocking layer. The Ag nanoparticles were incorporated into the exciton blocking layer by thermal evaporation. Compared with the conventional cathode contact materials such as Al, LiF/Al, devices with Ag nanoparticles incorporated in the exciton blocking layer showed lower series resistances and higher fill factors, leading to a 3.2% power conversion efficiency with a 60 nm active layer; whereas, the conventional devices have only 2.0-2.3% power conversion efficiency. Localized surface plasmon resonances by the Ag nanoparticles and their contribution to photocurrent were also discussed by simulating optical absorptions using a FDTD (finite-difference-time-domain) method.