The aggregation behaviors of two cationic surfactants, namely, N-dodecyl-N-2-hydroxyethyl-N,N-dimethyl ammonium bromide (C(12)HDAB) and N-dodecyl-N,N-2-dihydroxyethyl-N-methyl ammonium bromide (C(12)DHAB), were studied by measurements of density, conductivity, and steady-state fluorescence quenching. The aggregation number, the volumetric properties, and the standard Gibbs energy (Delta G(mic)(0)) of micellization were obtained, and the surface area (A(mic)) of the surfactant headgroups and the radius (r) of the micelles were estimated. It was found that substituting the methyl group with the hydroxyethyl group in the head of the surfactant led to the increase of the volume change of micellization and the decreases of the critical micelle concentration, association degree of the counterion, and the value of Delta G(mic)(0), while the aggregation number of those surfactants in aqueous solutions was little affected by introducing the hydroxyethyl group. It was also found that the values of A(mic) and r increase with the hydroxyethyl group.