Tension-induced structural changes in bovine Achilles tendon collagen at each level of the hierarchy structure were investigated by means of the X-ray diffraction method. In order to estimate the straining mechanism in a collagen fibril, three elementary models for molecular elongation and rearrangement of collagen fibril were proposed on the basis of the Hodge-Petruska model:​ [1] molecular elongation, [2] increase in gap region and [3] relative slippage of laterally adjoining molecules. The characteristic 67 nm D-period of a collagen fibril increases with applied force. A Hookean-type force-strain curve was obtained for the D-period while the force-strain relation for the tendon was non-Hookean. The relative intensity of third-order reflection of the D-period to that of the second-order one, I₃/I₂, decreased with the applied force. This decrease in I₃/I₂ indicates a decrease in the ratio of the overlap region of collagen fibril to the D-period, O/D, which was analyzed on the basis of the Hodge-Petruska model. Decomposition of the observed strain in the D-period, ϵD, into these three deforming modes revealed that the major contribution to ϵD originated from mode [1], molecular elongation. It was deduced that a fibril is mechanically composed of molecules connected serially to each other.