The present paper describes a different way of simultaneous determination of Fe(II) and Fe(III). This method was already applied in simultaneous determination of Cr(III) and Cr(VI) in tannery wastewater. And the previous experimental result shows that the method had good merits of good expansibility, small volume, high measuring accuracy and strong anti-jamming, So this procedure was tried to applied in the simultaneous determination of Fe(II) and Fe(III). The use of low pressure ion chromatography combined with flow injection spectrophotometry (LPIC–FIS) has been demonstrated for simultaneous online separation and determination of Fe(II) and Fe(III). The application of a low pressure ion chromatographic column (LPICC) packed with laboratory-made low pressure cation exchange resin (LPCR) was evaluated for the on-line separation of Fe(II) and Fe(III). Afterwards, hydroxylamine hydrochloride was used as the reducing agent. Finally, the phenanthroline was used as chromogenic agent in an online system for Fe(II) and Fe(III) determination. The main purpose of this work was to establish a simple, rapid and reproducible online method for simultaneous determination of Fe(II) and Fe(III). The proposed method has been applied to the determination of Fe(II) and Fe(III) in water samples with satisfactory results.