Clinical diagnosis in foot pathology requires repeated measurement of the geometrical relationship among bones. To analyze the complex structure of a foot and to capture a 3D pose of the bones, CT or MRI has been used in addition to the conventional radiograph. However, due to the lack of standard method for measuring 3D angles from 3D data of foot bones, clinical measurements are mostly obtained by measuring the distances and angles in 2D slices in a way similar to the measurements on a conventional radiograph image. In this paper, we propose an approach using a 3D foot template model for posture assessment to bring more reliable 3D foot analysis. To quantify robustly the bony relationship, we construct a template model with encoded articular surfaces on each bone which represents the neutral position of the foot as a reference for the measurement. The subject-specific foot posture is described by presenting angular changes and distance differences compared to the template model. We address the quantified postural difference between subjects in a unified frame-of-reference and perform an experimental study based on the proposed method. The resulting posture between a standing position and a supine position are clearly distinguishable in terms of both distance and angle. The proposed technique provides not only quantified measures of the foot posture but also guidance for finding meaningful differences between postures from different subjects.