Abstract: The technological parameters and methematical model of in-flighting droplet thermal process have been established by adopting Newton cooling, homogeneous nucleation and contineous growth model,and the mathematical calculation and analyses for ZA27 alloy have been conducted. Results show that under lower atomization pressure(~0. 7MPa), the cooling speed of droplets with a diameter of 20~80μm is in a range of 10⁶~10⁴K/s. The logarithm of undercooling degree to droplet diameter exhibits a linear relation. The bigger the size is, the smaller the maximum undercooling degree can be reached. The undercooling degree of droplets with a diameter of 20μm is about 0.2Tm. Once nucleus is formed in flight,the solidification will be completed within a shorter flying distance(0. 04m).