Social capital is one of the influential concepts in social science to understand contemporary societies. It has been found to influence many aspects of social life, directly or indirectly. It is also increasingly explored in relation to Information and Communications Technology (ICT). Nevertheless, social capital is a challenging variable to research, in part because of its multiple divergent definitions and measures. This paper presents and conducts a preliminary test of a model for understanding how ICT affects social capital. The model hypothesizes that the changes in social capital caused by ICT result from some degree of mobility in social interaction obtainable by using ICT over time. Principally, three types of mobility - temporal mobility, spatial mobility and contextual mobility - are identified. The preliminary test, using mobile phones as an example of ICT, was conducted in South Korea, which is one of the leading countries in mobile technology development. All three types of mobility are found to be important in explaining the impacts of mobile phones on social capital.