The process for preparation of iodo-substitued aromatic compounds by iodination of diazonium salt and potassium iodide was investigated in self-made PTFE-microreactor. The effects of the inner diameter of microchannel reactors, current, capacity resident time and reaction temperature on yield were performed. The optimum conditions were determined as follows: the inner diameter of micro-channel reactors 0.8mm, total flow rate 1004.8μL/min, reaction temperature 20℃, reaction time 75s. The yield of iodobenzene was 79.0% under these conditions. The STC in micro-channel reactor was 9.41?03 mol?m?3?h?1, more than 100 times of that in a regular reactor. The optimum reaction conditions were also applied in the preparation of other iodo-substitued aromatic compounds in microreactor.