The aim of this work is to study the effects of calcination condition on the properties of mesoporous silica, SBA-15 modified with phenyl group of organosilane. Functional group SBA-15 were synthesized and hydrothermally aged. The surfactant template was conventionally removed by calcination at varied temperature and duration. Thermogravimetric Analysis (TGA) was employed to characterize the pure and modified SBA-15. The TGA profile presented here would include modified SBA-15 functionalized using two different methods, directsynthesis and post-grafting. The characterization profile from TGA showed that the temperature of calcination has large influence in determining the effectiveness of surfactant removal and incorporation of alkoxysilane into pure silica whether by post-grafting or direct synthesis method. However, the duration of calcination does not significantly affect these characteristics. Besides, mesoporous structure of SBA-15 by post-grafting method shows higher degree of functionalization than that of direct-synthesis.