20 wt. % Co3S4/C nanoparticles as novel non-precious metal electrocatalysts were synthesized by two routes based on the in situ decomposition of cobalt carbonyl (Co2(CO)8) in p-xylene solvent: with the surfactants of oleic acid and trioctylphosphane oxide, and without surfactant directly using the supporting carbon (Vulcan XC-72) as the dispersing agent. The corresponding structure, thermal stability, electrochemical properties were investigated by means of powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), and rotating disk electrode (RDE) techniques. The particles synthesized without surfactant show the higher electrocatalytic activities toward oxygen reduction reaction (ORR) with the onset potential ca. 0.7 V vs. RHE in 0.5 M H2SO4 relative to those synthesized with the surfactants with the onset potential ca. 0.3 V vs. RHE, possible resulting from the surfactants blocking the active sites on the surface of the electrocatalysts. The synthesized 20 wt. % Co3S4/C nanoparticles demonstrate the thermal stability below 360 oC under air atmosphere and the electrochemical stability in the acid medium below 0.8 V vs. RHE.