In order to understand the molecular mechanism of relaxation phenomena in collagenous tissue, time-resolved, small-angle X-ray diffraction measurements were performed on bovine Achilles tendon collagen under creep. A tension-induced increase in the 67 nm period (D-period) was observed, and the strain in the D-period, ε_D, was found to be almost proportional to the external force per unit cross-sectional area (average stress) of the specimen. With an increase in ε_D, a change in the ratio of intensities of the third-order reflection peak of the D-period to that of the second-order peak was also observed. The increase in ε_D was decomposed into three elementary processes of D-period deformation, which are presented on the basis of the Hodge–Petruska model:​ (1) molecular elongation, (2) increase in gap region, and (3) relative slippage of lateral adjoining molecules. Up to 8 MPa of average stress, the contribution to ε_D originated mostly from only mode (1). At more than 10 MPa of average stress, modes (2) and (3) also contributed to fibril elongation. For ε_D by molecular elongation (mode (1)), the time dependence of the D-period change in the immediate response region is a sharply shaped step function, while the contribution to ε_D by molecular rearranging modes gives a slight creep nature at the immediate response region in the time dependence of ε_D. Because this creep nature is observed at the immediate response, it is related qualitatively to the KWW function in a stress-relaxation modulus of collagenous tissue observed in an immediate response region (Sasaki et al. (1993) Journal of Biomechanics 26, 1369–1376). The elementary process of KWW-type relaxation is concluded to be related to the tension-induced molecular rearrangement within a D-period.