Abstract: A method for the determination of azodicarbonamide (ADA) in flour by visual colorimetry was proposed based on the catalytic effect of ammonium metavanadate in phosphoric acid medium on the discoloration reaction of sodium indigo disulfonate oxidized by ADA. 1.0 mL of 150 mg·L⁻¹ ammonium metavanadate solution, 1.1 mL of 0.001 mol·L⁻¹ sodium indigo disulfonate solution, 3.0 mL of 85% (volume fraction) phosphoric acid solution and 4.5 mL of sample solution were added in the tube, and the reaction was carried out at 45 ℃ for 35 min. Using water as reference, the absorbance A of the solution at 610 nm was measured. The picture was taken by phone and the RGB value of the picture was extracted through the application program of color recognizer in smartphone. As shown by the results, linear relationships between the mass concentration of ADA and absorbance A or R/B value were kept within 40.0 mg·L⁻¹, with detection limits (3s/k) of 0.015, 4.518 mg·L⁻¹, respectively. Test for recovery was made on the blank sample by standard addition method, giving results in the range of 94.0%-114%, with RSDs (n=5) of the determined values in the range of 2.3%-6.8%.