Abstract: Micromixing in the twoimpingingstream reactor with different momentum ratios, different velocities, different distances of nozzles and different volumes of reactors was investigated by means of the consecutive competitive azo coupling reactions between 1naphthol and diazotised sulphanilic acid. Results show that greater momentum ratio can effectively improve micromixing status, and that there is a optimal characteristic resident time tRC when 0.03stRC.