The global patent market is growing year after year. As the patents market has grown up, the discussion of building and managing an optimal patent portfolio has continued. A patent portfolio is a strategic collection of individual patents and provides a strong synergistic effect that can compensate for the fundamental risks and uncertainties inherent in individual patents, however, there are not many studies on how to properly build the patent portfolio. Furthermore, in case a patent constitutes a patent portfolio as a strategic collection, the synergistic effect thereof should be reflected and the adequacy of the portfolio should also be evaluated. However, an appropriate evaluation method for carrying out this has not been proposed, therefore it is dependent only on the opinions of experts. Although a qualitative evaluation method is sometimes essential, it consumes a lot of time and cost, and the evaluation result may vary depending on the subjective opinion of experts, therefore reliability reconsideration is required. In this study, the relations between individual patents constituting a patent portfolio is analyzed based on quantitative factors that is obtained through patent literature, and proposes a method for optimal portfolio selection by proving the synergistic effect of portfolio composition from the mean-variance model.