Rapidly solidified Al-misch metal-Ni powders were produced by using gas atomizer. Studies about change of formation phase and solidification mechanism were performed. And optimal composition range for amorphous single phase formation was determined. We obtained amorphous powder which showed glass transition phenomena in the composition range of 15.7 at%≤(Ni+Mm)≤17.4 at%. In the other compositions, mixed phases consisted of α-aluminum and various metallic compound were obtained. It was found that cooling rate of powder which the particle size was below 20 ㎛ was at least two times faster than that of powder ranging from 20 ㎛ to 38 ㎛. Al_(84.2)Mm_6Ni_(9.8) and Al_(84.3)Mm_(6.9)Ni_(8.8) alloys had a good glass forming ability and were prospected as optimal composition for fabrication of bulky form. In addition, we observed the transition phenomena of nonequilibrium phase during crystallization by using XRD and in-situ TEM in these alloys.