The crystallization of amorphous alloys predestinates the microstructure and magnetic properties of the melt spun nanocomposite rare earth permanent magnets. Obeying the nucleation regularity, the model of the crystallization of amorphous alloys,namely, continually and explosively nucleation and growth. is put forward. The main factors affecting it were analyzed, such as melt-spun speed, the addition of alloys and the annealing sequence with or without the applied magnetic field, Especially, the re-distribution of the alloys during crystallization. On the bases, the ideas to gain ideal grain structure were discussed, for example, refining the grains by decomposition of metastable phase.