The purpose of the work described in this paper was to make a stress-strain curve for a collagen molecule and estimate Young's modulus of a molecule along the molecular axis. X-ray diffractometry was performed on bovine Achilles tendon in order to measure strain in the collagen molecule along the molecular axis as a response to a macroscopically applied force. By geometrical calculations and experiments, cross-sectional areas of a molecule and molecules in a tendon collagen fiber were determined. The applied force was translated to the stress and the stress-strain curve of the collagen molecule was constructed, which was found to be almost linear. Young's modulus of the molecule was determined to be slightly smaller than when determined by dynamic mechanical methods. The difference was considered to suggest the existence of a viscoelastic component within the molecule as well as the difference in the mechanical properties of collagen in different tissues. The expected viscoelasticity was speculated to be related to the hydrogen bond network in the collagen molecule.