Poly-silsesquioxanes have been numerously used to produce SiO₂ or Si_xO_yC_z ceramics through pyrolysis under different atmospheres. In this study, crosslinking and pyrolysis behavior of a commercial methyl-silsesquioxane, was examined using thermogravimetry analysis and a lower SiO₂ yield was observed after pyrolysis at low heating rates of 0.3 and 0.6 K/min under air atmosphere, namely 69.1 and 75.0 wt%, respectively. Using ceramic thermoplastic shaping techniques, a low heating rate, below 2 K/min, is usually required to remove processing additive (e.g., binders) gradually and avoid failures like blisters, pores, and cracks. A mixture of SILRES® MK and γ-Al₂O₃ was prepared to produce a mullite ceramic with Al₂O₃/ SiO₂ ratio of 60/40 wt%. As expected, the stoichiometric ratio of alumina to silica was altered using a heating rate of 0.6 K/min and mullite ceramic with 61.70/37.70 wt% of Al₂O₃/ SiO₂ ratio was obtained. By mixing above the crosslinking temperature of SILRES® MK, a constant ratio of alumina to silica could be achieved successfully, even at low heating rates. In addition, microstructure indicated the growth of needle-like mullite crystals with some amount of SiO₂-rich ternary amorphous phase (MgO–Al₂O₃–SiO₂) between the needle grains. Elemental analysis revealed that MgO sintering additive was mainly in the amorphous SiO₂-rich phase.