A Ginzburg-Landau theory is presented to study coupled first-order gas-liquid phase transition and second-order orientational phase transition in Langmuir monolayers. We show that the coupling between the two phase transitions has a strong influence on the orientational phase transition, making the polar order phase separated from the isotropic one. Such a phase separation will substantially suppress the onset of the transition and the growth of the polar order parameter, which accords with the existing experiments.