Direct in situ observation of the fine structure of the cationic lipid monolayer-DNA complex on a water surface has been carried out. by using an air-water interface X-ray reflectivity (XR) instrument. Interestingly, the thickness of the DNA layer was markedly thinner than the geometry of the cylindrical DNA molecule when the complex was deposited on the solid substrates; the thickness was determined to be 11 Angstrom by XR measurement in a dried state, while the diameter of the DNA molecule is about 24 Angstrom. However, the thickness in the complex on a water surface was estimated by in situ XR measurement to be about 25-28 Angstrom, which is comparable to the geometry of the DNA molecule. Thus, the anomalously thin thickness was due to some experimental treatments, such as deposition on a solid substrate and/or drying. The structure of the monolayer and monolayer-polymer complex on the solid substrates in a dried state is not the same as that on a water surface. The possibility of some dynamic fluctuation of its structure was also suggested. These results strongly indicate the importance of a direct in situ study such as by the XR technique for the structural study of the monolayer and monolayer complex on a water surface.