This paper shows effects of design factors for micromixers using the collision of fluid segments on mixing performance. Design factors are collision zone diameter, outlet channel diameter, collision angle of reactant fluids, and number of fluid segments for collision. We evaluate mixing performances of T- and Y-shape micromixers and the K-M mixer using the Villermaux/Dushman reaction. When the Reynolds number in the outlet channel, Re, is less than 200, the fluid segment size determined by only channel geometries, W-c, and Re are significant on mixing performance. From the results in this Re range, we correlate the by-product UV absorbance with the effective fluid segment size after collision, W-e = W-c/Re-n. The absorbance is linearly fitted with W-e. Thus, W-e expresses effects of total flow rate, outlet channel diameter, and number of fluid segments for collision on mixing performance. We can improve mixing performance by increasing flow rates as well as reducing channel sizes, leading to avoid extremely small channels to improve mixing performance and a high pressure drop in mixer channels. When Re > 200, high share rates applied to fluid segments also enhance the mixing performance. This indicates that high mixing performance and high throughput can be achieved simultaneously. (c) 2006 Elsevier B.V. All rights reserved.