Urea can enhance the aqueous solubility of surfactant CTAB (hexadecyltrimethylammonium bromide) when it shows the hydrotrope action.It will show the hydrotrope-solubilization action when the solubilized amount microemulsion and that of n-C5H11OH in O/W increased The mechanism of the hydrotrope-solubilization ac-tion of urea is the increase of the stability of W/O and O/Wmicroemulsion and structural transition from the lamellar liq-uid crystalline phase to the bicontinuous structure.