A new technique of Brewster angle ellipsometry was developed for quantitative evaluation of the Langmuir films extended on water surface. A laser beam withPpolarization is focused onto the film with its main axis at the Brewster angle &thgr;B. The angular distribution of the reflected light is observed by spatial scan of a photodetector. The scan along the direction ofPpolarization for the reflected light gives the angular dependence of the reflectivity ofP‐polarized light, which has a minimum at &thgr;B. The presence of a Langmuir film causes an increase in the minimum reflectivity, from which the quantities such as thickness or density of the adsorbed molecules are obtained. A Langmuir film of stearic acid in its condensed phase was used as a test specimen. The result showed an increase of 2.0×10−6in the minimum reflectivity, determined accurately to 10−7. The curve of reflectivity was in very good agreement with a prediction by a theory of geometrical optics made with the assumption that the index of refraction of the film is equal to that of bulk liquid and that the film thickness is equal to the molecular length. The spatial resolution on the film is ∼100 &mgr;m determined by the focused spot of the laser. This technique has the potential for optical absorption measurement in monolayer film. ©1996 American Institute of Physics.