A mesostructured precursor for KSW-2 with semi-squared mesopores was directly modified with chlorotrimethylsilane and chlorooctyldimethylsilane for the first time. The silylated products were characterized by powder XRD, IR, Si-29 MAS NMR, CHN analysis, and N-2 and water vapor adsorption measurements. Mesoporous silica KSW-2 prepared by calcination of the precursor was also silylated in a similar manner. The mesostructures of the precursor and of KSW-2 were retained after silylation. The characteristics of semi-squared mesopores remarkably changed; the hydrophobicity of the derivatives increased substantially after silylation. The BET surface area, the pore volume, and the width of the semi-squared mesopores decreased after silylation, and the degree of the decrease is larger for the octyldimethylsilyl derivatives than that for the trimethylsilyl derivatives. The numbers of the silyl groups grafted on the directly silylated derivatives are larger than those on the silylated KSW-2, In addition, calcination of the trimethylsilylated products yielded mesoporous silica with the same structure in which the pore wall thickness was increased because of the participation of the silyl groups to the silica network by oxidation.