Crystallization behavior of cordierite-based glass with excess SiO2 and Al2O3 (CM glass) was studied at the initial crystallization stage. Two kinds of glass-in-glass phase separation occurred during the heat treatment of glass powder. The secondary glass-in-glass phase separation, which consists of an SiO2-rich phase and an Al2O3-rich phase significantly affects the crystallization of the glass and the resultant microstructures of the crystallized glass. Mullite nucleated first in the Al2O3-rich amorphous phase which is no longer a stable glass-forming region. Metastable mu-cordierite crystallized in the SiO2-rich amorphous phase and grew rapidly. A fine-grained cordierite-mullite composite with mullite as the grain boundary phase was produced.