Atomization is an effective approach to obtain fine and spherical alloy powders. Undercooling is a important factor that can affect the property of powders during atomization. In this paper, the effects of powder size and cooling rate on the undercooling and microstructure of powders were investigated in the paper, and the relation between powder size, cooling rate and the undercooling was obtain through DSC. The result show that a small powder size and a low cooling rate will result in a large undercooling. Meanwhile, it was found that when the undercooling is large, the dendrite arm spacing of particles decrease. The smaller the powder size, the proportion of cellular grains increase, and the grain size of the powders will be even fine.