Abstarct: In this research, a simple and sensitive ultrasound-assisted surfactant-enhanced emulsification microextraction technique baxsed on the solidification of a floating organic droplet followe by high performance liquid chromatography with ultraviolet-variable wavelength detection was developed for simultaneous preconcenteration and determination of Chlorodiazepoxide and Alprazolam. The solvent, 1-undecanol, was used as an extraction solvent. For its low density and proper melting point near room temperature, the surfactant, CTAB, was used as an emulsifier to enhance the dispersion of the organic extraction solvent into an aqueous phase, which hastened the mass-transfer of the analytes. Some parameters (e.g., pH, the type and volume of extraction solvent, the type and concentration of surfactant, salt addition, and etc) that affect the extraction efficiency were optimized by experimental desing. Under the optimized conditions; a linear calibration curves in the range of 0.5-3.5 mg.L-1 for both drugs, with detection limits of 0.021 mg.L-1 for Chlorodiazepoxide and 0.02 mg.L-1 for Alprazolam. Also, the preconcentration factor of Chlorodiazepoxide and Alprazolam were obtained 109.4 and 105.0 respectively. The relative standard deviation for three replicate determinations of 0.5,1.0 and 3mg.L-1 of Chlorodiazepoxide were 3.3%,4.2% and 4.8% and Alprazolam were 3.4%, 3.9% and 4.08% respectively. This method was applied successfully for determination of these drugs in different real samples.