Micromixing in the submerged circulative impinging stream reactor (SCISR) developed by the authors is investigated with the Bourne's reaction scheme. The values measured for the impinging velocity, u(0), under the conditions of SCISR normal operation, only is of the order of 0.1m.s(-1), are much slower than that inferred, suggesting low power requirement for operation. The values of the characteristic time constant for micromixing, t(M), determined in the impinging velocity range of 0.184 m.s(-1) < u(0) < 0.326m.s(-1) are ranged from 192 ms to 87 ms, showing that impinging streams promotes micromixing very efficiently. The data follow approximately the relationship of t(M) proportional to u(0)(-1.5). A comparative study shows that the micromixing performance of SCISR is much better than that of the traditional stirred tank reactor. The tM values predicted with the existing theoretical model are systematically longer than those measured by about 2-3 times, implying that the regularity of impinging streams promoting micromixing is unclear yet.