The traditional arc discharge process for producing metallic nanoparticles was modified to be applicable for the direct preparation of oxide nanoparticles by using both conductive and insulative precursors. The cobalt oxide nanoparticles were synthesized by feeding Co and Co 2 O 3 into the air plasma. The as-prepared nanoparticles are spherical in shape with size in the range of 10 to 40 nm. The phases of CoO and Co 3 O 4 were detected in the samples prepared from the precursors of Co and Co 2 O 3 , respectively. Weak ferromagnetism and an exchange bias up to 17.1 mT were observed in these antiferromagnetic nanoparticles, owing to the presence of a large fraction of the uncompensated surface spins and possibly the local disordered regions.