Abstarct: In this study, a flow injection method was described for determination of trace amounts of thiourea in water. The method is baxsed on the inhibition effect of thiourea on the reaction between mexta Cresol Purple (MCP) and potassium bromate. The reaction is catalyzed by bromide ions. In the presence of thiourea an induction period was appeared in the reaction system, which is proportional to the thiourea concentration. The reaction was monitored spectrophotometrically by measuring the change in absorbance of MCP at 525 nm. The chemical and FIA variables were studied and optimized using the univariate and Simplex optimization methods. Under the optimum conditions, thiourea can be determined in the range of 0.100-13.0 µg mL-1. The limit of detection (3ϭ) for thiourea was 0.031 µg mL-1. The relative standard deviation for six replicate determination of 0.500, 5.00 and 12.0 µg mL-1 thiourea were 4.08%, 1.82% and 1.24%, respectively. The proposed method was applied to the determination of thiourea in water, orange juice and orange peels sample with satisfactory results.