The evolution of microstructures in two rapidly solidfied niobium microalloyed steels was studied. These alloys were rapidly solidified by two powder process techniques: nitrogen gas atomization and centrifugal atomization. It was found that in both powder processes, powder particles larger than 20㎛ diameter were martensitic, and that the nitrogen gas atomized particles solidified cellularly while those that were centrifugally atomized tended to solidify dendritically. Particles smaller than 1㎛ were not completely characterized because of wide variation in composition.