A micro-impinging stream reactor (MISR) at the size of similar to 1 mm has been built from commercial T-junctions and steel micro-capillaries, which has advantages of intensified micromixing and easy construction. By using a big-sized and very short outlet tube, the two injecting streams could impinge intensely within the limited chamber of the T-junction to achieve a quick mixing at the micro-scale. The effects of both operating parameters and geometric parameters of MISR, e.g., inlet jet Reynolds number (Re-j), volumetric flow ratio (R), outlet configuration, outlet length (L), and inlet diameter (d(i)), on the micromixing efficiency have been studied by experiments with the Villermaux-Dushman reaction as well as by computational fluid dynamics (CFD) method. Both experimental and numerical results agreed well and demonstrated that the inlet jet Reynolds number (Re-j) and inlet size played a key role for the micromixing performance. The outlet configuration also had a great influence on micromixing, but the effects of the length of outlet channel were insignificant. This work showed that MISR will have a great potential for both reaction and separation process intensifications. (C) 2014 Elsevier Ltd. All rights reserved.