The analysis of interference and efficiency are important phases in the design of planetary gear train. Because most planetary gear trains contain internal gear called ring gear, interferences between ring gear and planet gear should be analyzed in the step of design and manufacturing. Addendum modification coefficient, pressure angle, speed ratio between ring gear and sun gear are governing factors for interferences. In this paper, the interferences of 2K-H I type planetary gear train based on various planetary gear trains are studied. As that results, the ranges of addendum modification coefficients which would not lead to interferences is analyzed. Based on these ranges, theoretical efficiencies are investigated as 6 configurations of 2K-H I type planetary gear train, which is based on basic efficiency, and optimal addendum modification coefficients which generate the maximum efficiency of planetary gear train are presented. To prove results of theoretical efficiency analysis, experimentations are performed.