We have performed infrared visible sum frequency generation (SFG) spectroscopy of alkyl silane SAMs on quartz substrates prepared by chemical vapor deposition (CVD) of Octyldimethylmethoxysilane (ODMS) and Octyltrichlorosilane (OTCS). ODMS and OTCS have eight carbon (C8) chains with one and three precursors, respectively. The dependence of the structure of the alkyl chains on the precursor numbers was investigated. In the SF spectra, symmetric and asymmetric methylene stretching vibrations were observed at 28492852 and 29102918 cm-1 , respectively. The strongest peak at 2876~2879 cm-1 was attributed to the symmetric stretch of methyl end group. The peak intensity ratios of methylene stretching vibrations to symmetric methyl vibration of the ODMS monolayer were larger than that of OTCS. This result indicates that the number of gauche defects was reduced with the increase of the number of precursors. In the ODMS SAMs with one precursor the molecules are packed with low density due to the unstable anchoring and thus the van der Waals interaction between alkyl chains is weak. This should lead to the increased number of the gauche defects in the ODMS SAMs.