An approach to describe 3D urban form is proposed in this study. Central part of Nanjing is taken as study site.Firstly, quantitative attributes are proposed from urban geometry and its function, and they are compactness, elevation range, volume expansibility, diversity, dominance. Then, all the attributes are derived and spatial distributions are analyzed to reveal relationship between attributes and some geographical features. The result shows that the above five attributes closely correlate with land use; especially compactness, diversity and dominance reflect the feature of building configuration. Finally, a multi-spectrum image is produced by the optimal combination of attributes to classify urban form. The result shows that different blocks of different land use can be distinguished. This research reveals the spatial distribution of urban form properties and their relationships between some geographical features. This quantification analysis of #d urban form is a new way to study urban form from three-dimensional aspect.