The monolayer of a poly(vinyl alcohol) derivative having an azobenzene (Az)-containing side chain (6Az10-PVA) can induce an alignment transition of liquid crystal molecules in the monolayer by the trans-cis conformational change of the azobenzene unit (command effect). As a model system of this dynamic interface, the structural characterization of the mixed monolayer of 6Az10-PVA and a nematic liquid crystal molecule, 4'-penthyl-4-cyanobiphenyl (5CB), on a water surface was analyzed by in situ X-ray reflectometry. The depth density profile indicated that this hybrid monolayer in the trans-Az state consists of two distinct layers: a dense lower region and a less dense upper region having the same thickness of ca. 1.5 nm. This suggests that the rod molecules are stretched out in a direction perpendicular to the water surface where 5CB molecules are inserted on the lower side of the Az side chains. Light-induced structural modifications were clearly detected in the hybrid monolayer.