Abstract: The relative curve between the content of nitronium ions and the light density is mea-sured by means of model 721 chromophotomer, with cetyltrimethyl ammonium bromide as a standard sample，bromophenolblue as an indicator and trichloromethame as a solvent. Under the same condition，the cationic degree of cationic polymer is obtained by comparing the standard curve with those of light density of cationic polymer containing cationic nitrogenium pendent groups.

Key words: Bromophenolblue, Cationic degree, Cationic polymer, Optical density