We use gold nanospheres (Au NSs) to improve the performance of polymer organic solar cells. Au NSs with a diameter of about 5 nm or 15 nm were doped into the buffer layer of organic solar cells. We attribute the efficiency improvement to the size-dependent localized surface plasmon resonance (LSPR) effect of Au NSs, which can enhance the light harvest ability of active layer around the Au NSs, and increase the probability of the exciton generation and dissociation. Our results show that solar cells doped with 15 nm-diameter Au NSs exhibit significant improvement of the efficiency (from 1.99% to 2.36%), while solar cells doped with only 5 nm-diameter Au NSs did not give obvious improvement of the performance. Crown Copyright (C) 2010 Published by Elsevier Ltd. All rights reserved.