Abstarct: In this project, flow system with spectrophotometeric and electrochemical detectors was used for determination of organic and inorganic materials. For the first time, Flow Injection Analysis (FIA)-spectrophotometer system baxsed on simple kinetic reaction was designed for simultaneous determination of uric acid (UA) and ascorbic acid (AA). The FIA manifold was baxsed on the difference between the reaction rates of the AA and UA with Fe3+ in the presence of 1, 10-phenanthroline (phen). The absorbance of Fe (II)-phen complex was measured spectrophotometrically at 510 nm. The multi criteria decision making (MCDM) approach was employed in the optimization procedure for the selection of the most appropriate experimental conditions (alternatives) according to the require criteria. The weights of the evaluation criteria were determined using the analytic hierarchy process, entropy method, and compromised weighting (CW). The experimental conditions (alternatives) were ranked and selected by the technique for order preference by similarity to an ideal solution. Under the selected optimum conditions, the analytical signals obtained were linear in the ranges of 0.50-5.00 and 0.50-4.00 mg L-1 for AA and UA, respectively. The detection limits were 0.07 mg L-1 for AA and 0.14 mg L-1 for UA. The proposed method was also applied for the analysis of analytes in the blood serum, Vitamine C tablets, and tap water with satisfactory results.........