Abstract: Surface activity of two kind of ionic liquid-based imidazolium surfactants, "Octadecyl Methylimidazolium Tetrafluoroborate" and "Octadecyl Methylimidazolium Hexafluorophosphate" was investigated. Critical micelle concentration (cmc) of each ionic liquid was obtained by using empirical methods such as conductometry and tensiometry. The investigations showed that cmc of ODMImPF6 significantly are less than ODMImBF4. The results showed that hexafluorophosphate ion can reduces the surface charge of cations due to larger size and more ability in forming hydrogen bonding with water molecules, compared with tetrafluoroborate. In summary, these factors cause the reduction in repulsion forces between head groups and consequently cations can more easily be placed side by side to form micelles. Moreover, cmc of their mixtures is different from that of Clint's relation, because of synergistic effect.