The determination of the trace amount of mercury in aqueous samples was developed by nanocomplex formation of mercury dithizonate (Hg- (HDz)2 ) in water/ionic liquid media. The method is based on the nanocomplex formation of mercury dithizonate using water soluble ionic liquid. Mercury dithizonate complex is almost insoluble in water but its nanocomplex stabilized with [Hmim][Cl] in water. Thus, absorbance of nanocomplex in water was used for trace analysis of mercury in water samles. The optimal conditions such as; pH, reagent concentration, effect of ionic liquid were studied and analytical characteristics of the method were obtained. Under optimum conditions, dynamic range was obtained in the range of 5 to 1000 mg L-1 Hg (II). The detection limit was 0.5mg L−1 and the R.S.D. of the method was0.056%. A notable advantage of the method is the rapid determination of Hg (II) ions from trace volumes of water samples at ppb levels besides having both good accuracy and precision. Also, sensitivity of the method is enough for the determination of Hg(II) in variety of water samples.