Metallic cobalt nanoparticles are successfully obtained by the pyrolytic decomposition of CoC2O4 . 2H2O in the argon gas. The pyrolysates of CoC2O4 . 2H2O were investigated by TG-DSC and SEM. The results showed that there are two stages in the process of the pyrolytic decomposition of CoC2O4 . 2H2O in the argon gas. The crystal water in CoC2O4 . 2H2O was lost from 150 °C to 275 °C. CoC2O4 was pyrolysized into metallic cobalt powder from 300 °C to 500 °C. At the same time, the pattern of pyolysate of CoC2O4 . 2H2O was fined at 347.7 °C for 10 min. But, the particles of pyolysate of CoC2O4 . 2H2O were sintered into cobalt blocks at 500.0 °C for 10 min. Therefore, the conditions of pyrolytic decomposition of CoC2O4 . 2H2O were controlled if single cobalt powder was obtained at 500.0 °C..