Background and Objectives: Gold nanoparticles have found many applications in cancer diagnosis and therapy, drug and gene delivery and DNA and protein characterizations. Fungi are extremely good candidates in the synthesis of metal nanoparticles because of their ability to secrete large amounts of enzymes. The aim of this study was biosynthesis of gold nanoparticles by a fungus. Materials and Methods: The sampling was performed from the Ahar copper mine. The biomasses of isolated fungi were incubated with HAuCl4 solution in a shaker-incubator for 72 hr, and the strains that were able to produce gold nanoparticles were identified. The production of gold nanoparticles was studied with UV-vis spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). Results: Among the eight types of fungi that were isolated from the Ahar copper mine, only Rhizopus oryza was able to synthesize gold nanoparticles. The synthesis of gold nanoparticles was confirmed by observing the characteristic peak at 540 nm using UV-vis spectroscopy. The XRD analysis confirmed that the produced gold nanoparticles are in the form of nanocrystaline. Transmission electron microscopy images showed that Rhizopus oryza produces gold nanoparticles with good monodispersity in spherical and trigonal shapes both intra- and extracellularly. Conclusion: Fungus Rhizopus oryza is able to produce gold nanoparticles in the size range of 10-70 nm. This biologic method has the potential to replace chemical and physical methods currently used for gold nanoparticles production.