AbstractFibre flexibility is often anticipated to play a role in fibre flocculation phenomena. In this study, 3 thermomechanical pulps (TMPs) were sampled along a post-refining line. The only fibre morphological property that varied was fibre flexibility. These TMP samples were then tested for fibre flocculation and sheet formation tendency. The measurements clearly showed that fibre flocculation decreased and sheet formation uniformity increased with increasing fibre flexibility. The beneficial effect of fibre flexibility was larger at high consistency. These results support the elastic energy storage theory within flocs, which states that the more rigid the fibre, the stronger the flocs. Papermakers may take advantage of the beneficial effect of fibre flexibility on sheet formation through a careful tuning of post-refining.