Abstract: Cobalt particles were synthesized by the reduction method based on hydrazine hydrate complex and polyol. The obtained Co particles were characterized. The experimental results show that Co particles with different sizes, crystal types and properties could be obtained with different reduction temperatures. By constant temperature injection reduction, Co particles were transiently reduced in the solution of polyol solution at high temperature, which resulted in the explosive formation of the crystal nuclei. Restricted by the crystallization dynamics, the obtained Co particles are of nano-size (89—139 nm) with amorphous structure. By temperature programmed reduction, the supersaturation of Co particles in polyol is always low. The reduced Co particles grew slowly on the nuclei with finite number, and the obtained Co particles are of submicron-size (200—600 nm) with hexagonal crystal structure.