Structures of the self-assembled monolayer (SAM) of decanethiol (CH3(CH2)(9)SH, C10SH) formed on the reconstructed and (1x1)-Au(100) surfaces in 1 mM C10SH toluene solution were investigated by using scanning tunneling microscopy in air. The molecular arrangements of the SAM formed on the reconstructed and (1x1)-Au(100) surfaces were similar to the c(2x8) structure with (1x4) gold missing row, which is commensurate to the gold substrate, and the oblique structure, which is incommensurate to the gold substrate, respectively.