In this paper, we mainly investigated carbon-supported CoSe2 nanoparticles on the structure phase conversion under nitrogen and oxygen reduction reaction activity in 0.5 M H2SO4. The results of high-temperature powder X-ray diffraction and transmission electron microscopy techniques showed the structure conversion from orthorhombic at 300oC to cubic at 400oC and the increase of particle size from 2-3 nm to 5-10 nm. The electrochemical activity towards oxygen reduction reaction was improved from the orthorhombic to the cubic CoSe2 phase, for example, 3.5 electrons were calculated for the former and 3.7 electrons for the latter based on the average Koutecky-Levich slope at 0.2 V, 0.3 V and 0.4 V vs. RHE.