It has proven challenging to obtain collagen-mimetic fibrils by protein design. We recently reported the self-assembly of a mini-fibril showing a 35 nm， D-period like， axially repeating structure using the designed triple helix Col108. Peptide Col108 was made by bacterial expression using a synthetic gene; its triple helix domain consists of three pseudo-identical units of amino acid sequence arranged in tandem. It was postulated that the 35 nm d-period of Col108 mini-fibrils originates from the periodicity of the Col108 primary structure. A mutual staggering of one sequence unit of the associating Col108 triple helices can maximize the inter-helical interactions and produce the observed 35 nm d-period. Based on this unit-staggered model， a triple helix consisting of only two sequence units is expected to have the potential to form the same d-periodic mini-fibrils. Indeed， when such a peptide， peptide 2U108， was made it was found to self-assemble into mini-fibrils having the same d-period of 35 nm. In contrast， no d-periodic mini-fibrils were observed for peptide 1U108， which does not have long-range repeating sequences in its primary structure. The findings of the periodic mini-fibrils of Col108 and 2U108 suggest a way forward to create collagen-mimetic fibrils for biomedical and industrial applications.