Objective To investigate the property of collagen molecule assembling during the bone formation whencollagen molecules was excreted by osteoblasts; Methods Quantum Mechanics software Gaussian03 is applied inthe modeling of the stretch force-length curves of the cross-link between tropocollagen. Results The curve obtainedis non-linear , the start distance (energy optimized) of the two α carbon atoms is 10.05 ?. When the length reaches 130%of the start length, be 13.05 ?, an apparent turn appears on the force-length curve. The slope of the curve posterior tothe turn is much greater than that of the anterior; The maximum length is 16.05 ? when fracture take place; The stretchforce reaches the highest value 5622 pN; Break take place between α carbon atom and β carbon atom of the hydroxylysineresidue of the cross-link; Conclusion The mechanical characteristics of the cross-link predicted here by quantummechanics modeling need experimental corroboration.