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Nickel-based superalloy powders have been produced by high pressure argon gas and nitrogen gas atomization, respectively. The microstructural characterization of nickel-based alloy powders has been performed with the help of a scanning electron microscope, equipped with an EDS microanalysis unit. Based on a Newtonian cooling model, the flight speed and the cooling rate of two kinds of alloy droplets are calculated. The results show that the droplet cooling rate, which depends on atomization medium and droplet size, has an effect on the solidification microstructure. For argon-gas atomized powders, the developed dendrite structure is predominant at a lower cooling rate and a mixed microstructure composed of dendrite structure and cellular structure is observed at a higher cooling rate. For nitrogen-gas atomized powders, the dendrite structure is predominant at a lower cooling rate and a full cellular structure can be observed at a higher cooling rate. According to calculation, the cooling rate of argon-gas atomized droplets is in a range from10000K/s to 424000 K/s , while the cooling rate of nitrogen-gas atomized droplets is from 10000K/sto 480000 K/s . The cooling rate increases with decreasing of droplets diameter. Two kinds of atomizing gases have a slightly influence on the cooling rate of droplets. The elements such as Cr, Co, W, Ni and Al are rich in the dendrite axis, while Ti element is rich in the inter-dendrite region.