A fast, simple, and efficient method for extraction, preconcentration, and determination of thorium in water samples with acceptable recoveries based on flotation-assisted homogenous liquid-liquid microextraction combined with inductively coupled plasma-mass spectrometry (ICP-MS) is presented. Various parameters affecting the extraction efficiency were optimized by Box-Behnken design. Under the optimum conditions (concentration of dimethyl vinyl phosphonate = 2.4 x 10(-4) mol/L, pH = 6.5, n-heptane = 150 L, and acetonitrile = 0.5 mL), the calibration graph was linear in the range of 10.0-400.0 ng/L for thorium. The limit of detection of this method was 2.62 ng/L, and the enrichment factor was estimated to be 136 for thorium.