The interaction between I-3 and malachite green（MG）and its application for determining trace dopamine by resonance light scattering（RLS）method were studied.It was found that malachite green could react with I-3 to form an ion-association complex（MG（I3）2）with the composition ratio of 1∶2 by electrostatic forces in citric acid–sodium citrate buffer solution（ pH 4.5）,in the presence of polyvinyl alcohol（PVA）and p-octyl polyethylene glycol phenyl ether（OP）.Dopamine could convert I-3 to I-,resulting in the decrease of resonance light scattering intensity of the malachite green-I-3 system.Based on this,a new simple method with high sensitivity and good repeatability was developed to determine dopamine by resonance light scattering.Under the optimal conditions,the calibration curve was linear in the range of 2.0×10-7-4.5×10-6 mol?L-1,with a correlation coefficient of 0.999 1 and a detection limit（3S/k）of 9.5×10-8 mol?L-1.The proposed method was used to determine dopamine in injection and human serum samples with mean recoveries of 100.1%-102.4% and relative standard deviations of 0.41%-4.8%.