Polycarboxylic acids have been used as crosslinking agents for wood pulp cellulose for improving paper wet strength. Our previous research showed that low-molecular-weight polymeric carboxylic acids are effective in improving paper wet strength retention and reducing its flexibility. In this research, we compared two polymeric carboxylic acids, that is, poly(maleic acid) (PMA) with M-n of 800 and poly(methyl vinyl ether-co-maleic acid) (PMMA) with M-n of 1,130,000 for improving paper wet strength. The kraft paper sheets were treated at a 2% acid level and cured at different temperatures. The dry strength, wet strength, and folding endurance of the treated sheets were measured. We found that PMA and PMMA have comparable effectiveness in improving paper wet strength and wet stiffness. However, the treatment with PMA increases paper brittleness and severely diminishes paper folding endurance, whereas the treatment with PMMA. increases both the dry strength and folding endurance by enhancing the paper＇s toughness. This striking difference in the performance of the treated paper is attributed to the different nature of the crosslinkages formed on the sheets.