A crown-ether-type colorimetric chemosensor EDDAJ (9,9 0 -((1E,11E)-5,8-dioxa-2,11-diazadodeca-1,11-diene-1,12-diyl)bis (2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-8-ol) was synthesized. EDDAJ could detect Fe 3+/2+ by a color variation from colorless to dark green in aqueous media. Detection limit of iron ion was 0.43 μM. Complexation ratio of EDDAJ to Fe 3+ was determined to be 1:1 with the results of Job plot and ESI-mass. Based on Job plot, EPR, and ESI-mass experiments, the probing mechanism of EDDAJ to Fe 3+/2+ was demonstrated.