In this study, the simultaneous effect of SiO 2 nanoparticles and cetyl trimethyl ammonium bromide (CTAB) as an eco-friendly cationic surfactant on drop size of toluene in water dispersion is investigated. For this purpose, a single stage horizontal mixer-settler equipped with two series of 4-pitched blade turbine impellers with 45° blades on one shaft was employed. The influences of the presence and concentration of CTAB and also hydrodynamic parameters on drop size in the system containing a fixed amount of SiO 2 are studied. Results indicate that applying SiO 2 and CTAB together leads to about 52% decrease in mean drop size compared to the system containing SiO 2 alone. Furthermore, addition of CTAB into the systems containing SiO 2 diminishes the influence of the hydrodynamic parameters on D 32 . Eventually, in order to predict the mean drop size in the simultaneous presence of SiO 2 and CTAB, modified correlations are provided for the first time wherein the surfactant concentration introduced as a new independent parameter.