We have investigated the role of gold nanoparticle incorporated in the bulk-heterojunction active layer of poly(3-hexylthiophene) (P3HT)/fullerene. Since the performance of this organic photovoltaic device is strongly related to the charge carrier transport mobility of bulk heterojunction that arises from the molecular ordering of P3HT semiconductor, we have set the range of nanoparticle concentration to the infinitesimal level, which does not affect the thermally-induced ordering and conductivity of P3HT/fullerene composite film. While the increase of power conversion efficiency is mainly attributed to the enhancement of photocurrent, both the fill factor and the open circuit voltage of photovoltaic device were not sensitive to the concentration of gold nanoparticles, which does not coincide with the previous report on the case of poly(3-octylthiophene)/fullerene/nanoparticle mixture. We suggest that an enhancement of the photoinduced short-circuit current and efficiency can be explained by the increased absorption of nanoparticle-polymer composite film with the condition of the consistent molecular ordering of donor-acceptor bulk-heterojunction. At the weight fraction of 6.25 x 10(-8) for gold nanoparticle/P3HT, about 50% of enhancement (i.e. from 1.43 to 2.17%) was obtained in the power efficiency. (C) 2008 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.