An increased use of teams of actors within complex systems has led to the emergence of various approaches for the assessment of different features associated with team performance. Over the last two decades, the performance of teams in complex systems has received considerable attention from the human factors community, and a number of methods have been developed in order to assess and evaluate team performance. The purpose of this paper is to propose a methodology for measuring team efficiency of operators in the advanced main control room of Korean nuclear power plant. Team efficiency is an index which is estimated of gabs between individual performances and team performance. The index was developed to compare among teams through past all performance measurements.