A theoretical model is presented to study the phase transition of molecular orientation in a Langmuir monolayer on air-water interface. The polar molecules are treated as rodlike dipoles with three types of interactions; short-range steric repulsive interaction among the molecules, polar interaction between the molecules and water surface, and dipolar interaction among the molecules. The orientational order parameters, [cos theta] and [(3 Cos(2) theta - 1)/2], are calculated as functions of the molecular density and temperature. It is shown that compressing the molecular area will lead to a continuous phase transition of the molecular orientation.