This article is concerned with a proposed simple method to design an exter-nally pressurized trapezoidal and rectangular screw and nut mechanisms. The general lubrication assumptions are applied to the governing equations and continuity equation of the lubricant. The reduced equations are then used in an assumed double film annular bearing which is considered to be .equivalent to one turn of the hydrostatic screw. It is found that the performance characteristics of a hydrostatic screw such as power consumed ,volume flow rate, and static stiffness can be estimated in an easy manner. Furthermore, the optimum operations conditions can easily be determined.