N-chloro-anionic-polyacrylamides (N-Cl-A-PAM) were prepared by N-chlorination of anionic polyacrylamide (A-PAM) with NaOCl at 25℃, and applied to improve paper strength. By the addition of N-Cl-A-PAM (0.5% on pulp) containing 10 molar% of carboxyl groups at pH 4.5, the dry tensile strength of formed sheets was increased at the level of 37%, and wet tensile strength was 12.5 times larger than those of the blank sheets. N-Cl-A-PAM were much more effective on the improvement of paper strength than A-PAM. The content of carboxyl groups in N-Cl-A-PAM greatly influenced their effectiveness. The N-Cl-A-PAM with 10 molar% of carboxyl groups gave the maximum improvement of paper strength at pH 4.5, while the N-Cl-A-PAM containing 5 % of carboxyl groups was the most effective at pH 7.5. The higher the retention of N-Cl-A-PAM were, the larger the improvement of paper strength by the N-Cl-A-PAM were. It was suggested that the formation of ester bonds between hydroxyl groups on fibers and N-Cl groups of the N-Cl-A-PAM under acidic conditions was the one of the main reasons for the increases of paper strength. Under alkaline conditions, the improvement of paper strength was attributed to the formation of network by N-Cl-A-PAM themselves. It is found that improvement of wet tensile strength by N-Cl groups of N-Cl-A-PAM decreased as the content of -COOH increased. This is explained by the decrease in reactivity of N-Cl group which connects with carboxylic group through neiboring group participation. The effect of N-Cl group was larger in acidic solution than that in alkalic solution.