This paper focuses on flow lines composed of multiple parallel-machine stages. The system is similar to a classical flow line, the only difference being that a given stage may consist of parallel machines. A method for modeling and analyzing this type of flow lines is presented. The technique used amounts to replacing each parallel-machine stage by a single equivalent machine in order to obtain a classically-structured flow line with machines in series. Equations for computing the parameters of equivalent machines are submitted. These parameters can then be used to carry out performance diagnosis of the corresponding stage, or as input data for existing analysis methods to evaluate the production performance of flow lines. The method presented in this paper has been successfully implemented and tested in the industry over the past few years.