The potential of the new cellulase Cel9B from Paenibacillus barcinonensis for the modification of cellulose flax fibers was for the first time assessed in this work. Cel9B treatments were found to enhance the development of properties during the refining of fibers, the precise effects depending on the enzyme dose used. Scanning electron micrographs showed the treatments to alter flax fiber surface and cause defibrillation, peeling and fiber wall removal. Zero-span tensile strength was determined to elucidate the changes in intrinsic resistance of cellulose fibers and solubilization of carbohydrates evaluated by HPLC. Low Cel9B doses resulted in improved tensile strength in handsheets and can thus be used to save energy in the refining process. Also, on equal tear resistance, handsheets had a higher tensile index. High doses of Cel9B raised the collapsibility and bonding potential of cellulose flax fibers and produced heavier handsheets. Cel9B also improved air permeance at low refining intensities. The results obtained in this work are useful in understanding the effects brought by cellulase Cel9B on flax fibers and provide a proof of a biotechnological platform for the development of added-value fiber products