AbstractCarboxylic acid groups were introduced onto thermomechanical pulp (TMP) long fiber surfaces by 2,2,6,6,- tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation in the present study. The number of introduced carboxylic groups was closely related to the NaClO dosage, and more flexible fibers with lower curl and kink index were generated. Lignin was dissolved during the TEMPO-mediated oxidation, and its content was 24.2% with a carboxylic content of 1444 mmol/kg, in contrast to the control, which had 33.6%. Meanwhile, significant decreases in uncondensed lignin and β-O-4 lignin were observed during the TEMPO-mediated oxidation. The generation of carboxylic acid groups enhanced both the tensile and burst strengths of oxidized TMP significantly, and the value was 70% higher than the control with a carboxyl content of 1444 mmol/kg. However, side reactions during TEMPO-mediated oxidation led to a decline in intrinsic fiber strength, which may have contributed to the decline in paper tear strength.