A separation/preconcentration procedure for nickel in urine and water samples is proposed prior to its flame atomic absorption spectrometric determination (FAAS). The procedure is based on the adsorption of nickel on Diaion HP-20 resin as 1-nitroso-2-naphthol (NN) complex. Different factors including pH, sample flow rate, sample volume, reagent amount and matrix effects were examined. Nickel as NN chelate at pH 8.0-13.0, contained in a sample volume of 25-500 mL can be preconcentrated by using 0.7 g Diaion HP-2.0. The adsorbed nickel was eluted from the column by using 1M HNO3 in acetone. The nickel contents of a stream sediment reference standard material (GBW 07309) was determined by FAAS with satisfactory results. The method was applied to urine, drinking and industrial waste water samples.