The crystallization processes of MgO-B 2 O 3 -SiO 2 slag have been investigated using two methods of heat treatment. Phase separation, which is similar to grape clusters, was observed when the amorphous slag was heat-treated at 800 °C, however, dendrites which are found to be 2MgO · B 2 O 3 , were formed at 850 °C. The micrographs of phase separation and the dendrites show fractal characteristics and their fractal dimensions were calculated using a density-density correlation function to be 1.90 and 1.63, respectively. In order to reveal their formation mechanisms, phase separation and the growth for 2MgO · B 2 O 3 were simulated using the Monte-Carlo method based on spinodal decomposition, nucleation and growth, diffusion-limited aggregation (DLA) and random rain (RR) models in a computer. The simulation results show that phase separation corresponds to nucleation and growth, but the growth for 2MgO · B 2 O 3 depends on the DLA.