Abstarct: In this study a simple and sensitive method of dispersive liquid- liquid microextraction (DLLME) for preconcentration of cobalt prior to ,it’s determination by flame atomic absorption spectrometry was proposed. The analytical procedure baxsed on the formation of a complex between Co (ІІ) with hydroxy naphthol blue (HNB) as a complexing agent. After extraction of the complex by carbon tetrachloride as extraction solvent , cobalt concentration in organic phase was determined by flame atomic absorption spectrometry. The effect of different parameters such as pH, kind and volume of buffer, kind and volume of extraction solvent, kind and volume of disperser solvent, kind and volume of dilution solvent, concentration of ligand, volume of aqueous phase, extraction time, ionic strength and rate of centrifuge were investigated. Also, the effects of foreign ions were investigated. Under the optimized conditions, a linear calibration curve in the range of 0.002- 0.1 mg.L-1, with detection limit of 0/149 µg.L-1 and enrichment factor of 44.2 were achieved. The relative standard deviation for six replicate determinations of 0.020, 0.030 and 0.040 mg.L-1 of cobalt (ІІ) were %4.95, %3.45 and %4.87, respectively. This method was successfully applied for determination of cobalt(ІІ) in the spiked water samples