Composite pipe has been used in transporting corrosive fluid in many chemical processes in the petrochemical and pulp and paper industries. It is often subjected to external pressures due to process conditions (vacuum), water hammer, pump suction, and differences in elevation head. Most composite pipe designers are currently using traditional metallic pipe standards which do not take into account the anisotropic nature of composite materials. An analytical model for the buckling of orthotropic composite pipe under external pressure has been developed in this study. In this study, first-order laminated anisotropic plate theory was used to construct models of the kinematic and constitutive behavior of the composite cylinders. The Ritz Method was then applied to determine the buckling load under external pressure. Results obtained from the developed model were compared with the theory derived by Flgge for the case of metallic pipe. Due to the inclusion of shear deformation, the new model gave lower buckling loads than Flgge's results especially for the case of thick-walled pipe. Good agreement was found when comparing the developed model with experimental results provided by Fibercast Company following ASTM D 2924 for centrifugally cast composite pipe.