Recently 3D GIS and its applications are getting attention increasingly as various ubiquitous-related techniques for 3D spaces are being developed. Although they call for quantitative measures such as accessibility, most accessibilityrelated studies are limited to 2D networks, not 3D indoor space levels. In this paper, we develop an accessibility index applicable to 3D models. We first examine the theory of Space Syntax which has been developed and used to measure the connectivity or relationships between spatial segments in urban or architectural environments. Then, we expand the principle to a more general form so it can be applied to both street and indoor space levels. We incorporate different types of impedances in moving between places including distances, turns and transfers between floors into the traditional Space Syntax that measures the spatial depths solely based on the structural forms. Finally, we illustrate the use of the proposed measure comparatively using a campus building.