We synthesized size-controlled zeolitic imidazolate framework (ZIF-8) nanoparticles using a T-type micromixer. The nanocrystallization of ZIF-8 proceeds by a rapid coordination reaction of Zn2+ ions and 2-methylimidazole (2-MeIM) in a highly concentrated solution. Therefore, a rapid mixing of the raw materials is required to control the size and morphology of ZIF-8 nanoparticles. First, we prepared ZIF-8 nanoparticles at various flow rates of raw materials to investigate the mixing performance of a T-type micromixer. The size of the ZIF-8 nanoparticles decreased with an increase in flow rates at Reynolds number (Re) < 2000 (regarded as laminar flow). However, the size and shape of the resultant nanoparticles did not depend on the flow rates at Re > 2000 (transition state between laminar flow and turbulent flow), demonstrating the high mixing performance. Further, we systematically examined the effects of temperatures and [2-MeIM]/[Zn2+] ratios on the resultant ZIF-8 nanoparticles to elucidate the formation mechanism and to optimize the conditions for the synthesis of smaller ZIF-8 nanoparticles. Through a detailed analysis, we concluded that the requisite conditions for the preparation of smaller ZIF-8 nanoparticles are. lower temperatures and higher [2-MeIM]/[Zn2+] ratios, which affect the nucleation process and the particle growth process, respectively. Finally, we examined the adsorption properties of the resultant smaller ZIF-8 nanoparticles. We found that the amount of adsorbed N-2 gas for the ZIF-8 nanoparticles synthesized by our method is higher than the amount for a conventional ZIF-8 sample, and the adsorption rate is faster because of the nanocrystallization. (C) 2012 Elsevier B.V. All rights reserved.