When a buckle is initiated in a pipe subjected to external pressure, it will propagate along the longitudinal direction of the pipe if the external pressure is greater than its buckle propagation pressure. For a steady state condition, the propagation is simply considered as the translation of the buckle along the pipeline. This paper presents a unique approach to determine the length of the transition zone in a buckle propagating pipe by analyzing the mechanism of postbuckling of the pipe subjected to the external pressure. Buckling is considered to occur locally in the shell, spreading over a certain length along the longitudinal axis of the shell. The governing equations are derived from the postbuckling theory. Approximate solutions are obtained from the Ritz method, using a plausible function of the flexural displacement created based on Timoshenko's ring solution of the transverse collapse mode. The postbuckling equilibrium path shows that the pipeline experiences unstable collapse until the two opposite points on the inner surface contact each other. The length of the transition zone is found to be proportional to the ratio of (radius)3/2/(thickness)1/2 and is hardly affected by the material properties. The analysis is performed by comparing the obtained results with well-established predictions.