A spectrophotometric and first derivative spectrophotometric determination iron(III) is carried out with 1-nitroso-2-naphthol as complexing reagent in aqueous phase using non-ionic surfactant Tween 40. the iron is determined in the range 1.7-120ng mL-1 with detection limit (2α) of 1.7ng mL-1. The molar absorptivity and Sandell’s sensitivity are 1.69 x (104 mol-1 cm-1) and 7.9 ngcm-2at 446 nm. The optimum pH of the complex is 1. the critical micelle concentration (cmc) of Tween 40 is 5%. Adsorption studies in the first derivative mode is carried out to determine the adsorption maximum of the complex for higher sensitivity and to overcome interference due to the presence of certain metal ions. The present method is compared with that of AAS and no significant difference is noted between the two methods at 95% confidence level. The method has been applied for the determination of Fe (III) in industrials waste water and steel samples .