This paper focuses on the production of metal nanoparticles in a dielectric barrier discharge (DBD) reactor at atmospheric pressure. In the developed reactor only one electrode is provided with a dielectric (alumina), while the point-shaped counter electrode is not covered and is accessible to the filamentary discharges (asymmetrical DBD). The metallic particles are formed from the material of the open counter electrode, on which the filamentary discharges lead to the formation of craters. A high number of larger nanoparticles (with diameter of some tens of nanometers) are observed in the cold reactor at the beginning of the discharges. After the achievement of the thermal equilibrium in the reactor mainly very small metallic nanoparticles are produced with constant size and number concentration. The produced metallic particles in thermal equilibrium are crystalline and not agglomerated. The mean diameter of the produced particles is about 4 nm. For given operating conditions of the reactor (fixed electrode gap and gas flow rate) the electrode material has almost no influence on the size of the produced particles, but it determines the number concentration.