Human activity space is well-known to be related to his geographic, social-culture position, build environment and modal usage. The interrelationships between these observed and unobserved factors shape a person's spatial usage and visited activity locations. This study applies the structural equation modeling approach to identify the direct and indirect effects of these factors on the size of an individual's activity space. The data is based on the recent mobility survey for three European Institutions: the European Investment Bank and the Court of Justice of the European Union in Luxembourg city and the Council of Europe in Strasbourg (France). The empirical analysis shows that the size of a person's activity space is mainly explained by the build environment and less related to the socio-demographic variable when a workplace is controlled. The suggested structural equation model provides a flexible framework to investigate empirical effects of these factors on the activity space.