Objective: Use of microorganisms for the synthesis of nanoparticles emerges as an eco-friendly and exciting approach. Fungi have many advantages for nanoparticle synthesis compared with other organisms, for example, due to rich intracellular and extracellular reservoir of enzymes. In this study, Geotrichum sp. was studied for silver nanoparticle synthesis. Materials and Methods: The fungus Geotricum sp. was isolated from the soil and confirmed with macroscopic and microscopic study. It was grown in Saboraud dextrose agar medium (Merck, Germany) with pH=7 at 25 °C for 96 hours. Then it was washed by sterile normal saline, and fungal mycelia were separated from solid medium. The collected mycelia were washed five times with phosphate buffer saline and centrifuged at 5000 rpm for 15 minutes. The pellets were collected to obtain about 1 g of wet cells, and finally resuspended in sterile normal saline in a test tube. Then, 2×103 M aqueous AgNO3 (Merck, Germany) solution was added. Experiment tubes were incubated at room temperature for a period of 15, 30 and 45 days. Then this suspension was centrifuged at 5000 rpm for 5 minutes, and supernatant was studied. The produced silver nanoparticles were analyzed using UV-visible spectroscopy, atomic force microscopy (AFM), Fourier transform infrared (FTIR) analyses, and evaluation of change in color of medium. Results: Silver nanoparticles in the range of 30 nm to100 nm were synthesized in 15 days extracellularly. The synthesis was verified by methods UV-visible spectroscopy, AFM, and FTIR. Conclusion: All data show that this organism can synthesize silver nanoparticles like other microorganism that do the same. This is the first record of nanoparticles synthesis by this species. A major drawback for this type of production is that it is time-consuming. The advantage of using this fungus is its rapid growth and also its little need for rich nutrients.