Three enzymes, mannanase, xylanase, and cellulase, were applied for hydrolysis of thermomechanical pulp (TMP) primary discharge prior to PFI refining, aiming to study the effect of enzymatic hydrolysis on the required number of PFI revolutions. The quantity of reducing sugar was used as an indicator for enzyme hydrolysis efficiency. Then, under the optimized enzyme loading, treated and un-treated pulp were refined with different PFI revolutions. Subsequent fiber characteristics, such as fiber length and fines content were examined. Under the optimized enzyme loadings and a given 20000 PFI revolutions, in comparison with the control pulp, mannanase and xylanase pre-treatment could save PFI refining revolutions by 20% and 25%, respectively. There was no significant energy savings for the cellulase-treated pulp. During the hydrolysis, the enzyme broke down TMP fibers into shorter pieces and yielded more fines than the control pulp. Among the three enzymes, cellulase showed the highest efficiency in fiber breakdown, mannanase in the middle, xylanase the lowest. Longer hydrolysis time (more than one hour) had no evident effect on the pulp freeness reduction and reducing sugar production. Among the three enzymes, under the optimized enzyme loading, cellulase was the most efficient for enhancing production of reducing sugars.