Axial invariance of production performance encountered in a horizontal tubular reverse osmosis has been analyzed by assuming that the free convection is the dominant transport mechanism. The governing equations, which had been deduced from dimensional analyses used in heat transfer problem with a large Prandtl number, have been solved approximately by the integral method. The numerical computations show that the system performance in terms of concentration polarization and wall permeation velocity, even if axially constant, is better at the top of the tube than at any other circumferential positions. The effects of system parameters such as feed concentration level, operating pressure and tube radius can also be explained from the results obtained.