Collagen fiber, a major component of the extracellular matrix in the human lung, is crucial in maintaining the lung structure mechanically. It is necessary to study the collagen fiber orientation which the mechanical function is closely related to. In the present study the collagen fiber orientation in the lung was quantitatively measured by Osaki's microwave method. We succeeded in preparing sheet samples cut in a coronal direction from the lung for the measurements. It was found that the collagen fibers were, on average, orientated parallel to the longitudinal axis of the spine. The void spaces in the lung sample observed using an optical microscope was not circular but ellipsoidal. The direction of the long axes of ellipsoidal voids coincided with that of the collagen fiber orientation. The results suggested that collagen fiber orientation is closely related to the respiratory movement of the human lung. Anat Rec, 296:846–850, 2013. © 2013 Wiley Periodicals, Inc.