The purpose of this research is to present the major factors which determine dominant design of Smartphone OS and to verify a logical causal relationship with them and user satisfaction. To achieve this goal, OS compatibility, OS enhancement, Installed base, and network effect were presented as determinants of Smartphone OS which serves as the basis for determining dominant design and their logical causal relationship with user satisfaction was empirically analyzed. The results of the research showed that OS compatibility and OS enhancements had a positive effect on the expansion of user base. Of the two factors, OS enhancement showed that it does not only expand the user base but also acts as the cause for the fortification of the network effect. Furthermore, it was shown that OS enhancement had a direct positive effect on user satisfaction. The findings of this research are significant in its academic contribution as well as its potential for practical application by presenting the logical causal relationship in the process of determining OS dominant design an area which was previously untouched by empirical testing.