Abstract: The paper presents a model for clustering homogeneous firms according to their operationefficiency over a certain time period. The firm efficiency is studied by the Data EnvelopmentAnalysis (DEA) methodology, which is based on solving optimization problems and permits one tocompare firms taking into account many factors of their operation. At the second step of theanalysis, the time series of firm efficiency estimators obtained by DEA modeling are used tocluster firms and find stable partitions of the set of firms.