Machinable mica glass ceramics with more ZnO and B2O3 showed the phase separation by spinodal decomposition. the size of two phases formed by phase separation is in micron meter order. The nucleation and growth of crystal is performed through the diffusion of atoms. Another kind of materials with no ZnO and B2O3 addition behaves different mechanism of phase separation in terms of nucleation and growth. Many fine particles are obtained after the phase separation , the size of which is less than 100nm in diameter. Subsequently nucleation and growth in the crystallization is accomplished by aggregation and unification of the granular phase precipitation. The mechanism of the aggregation and unification nucleation-growth is different from one controlled by atom diffusion in nature. The different mechanisms in crystal nucleation and growth, caused by the composition changing.