The influence of PFI mill beating variables on the characteristics of fibre development of a high yield (c. 90%) jack pine chemimechanical pulp have been investigated. The beating variables were pulp concentration, (p.c.) clearance between the roll and housing, and relative speed. It was observed that beating at 10% p.c. resulted in severe fibre shortening, particularly when a nominal zero gap was used. On the other hand, high p.c. (30%) beating with a higher than the normal relative speed improved fibre quality and improved the mechanical properties of handsheets.