Abstract: This paper presents a study of the mechanism for thermal decomposition of SiC fibre reinforced glass-ceramic matrix composite and the relation to crystallization by TG-DTA-MS coupling technique. The thermodynamic and mas spectroscopy analysis for the composite SiC of forming carbon layer on the interphase of the composite SiC fibre/LAS are presented. A comparison between experimental phenomena and theoretical analysis for the thermal decomposition behaviour of both composite SiC fibre/LAS (crystallized and without crystallized) is also made.