A comparative study in molecular arrangements of the n-octadecyltrichlorosilane (OTS) monolayer prepared by the Langmuir method and the chemisorption methods were carried out on the basis of grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity (XR) measurements. The OTS molecules in the Langmuir OTS monolayer uniformly tilt ca. 8-10 degrees to the surface normal and packed in a hexagonal lattice with the (10) spacing of 0.412 nm. On the other hand, the OTS molecules in the chemisorbed OTS monolayer tilt ca. 15-17 degrees to the surface normal and also crystallite orient randomly in the two-dimensional plane. The average magnitude of the (10) spacing of the chemisorbed OTS monolayer was ca, 0.417 nm. Direct evidence that the packing density of the Langmuir OTS monolayer was higher than that of the chemisorbed OTS monolayer was obtained by GIXD and XR measurements for the first time.