In the present study, we synthesized zeolitic imidazolate framework-8 （ZIF-8） particles, which is a family of porous coordination polymers （PCPs） or metal-organic frameworks （MOFs）, by mixing a zinc nitrate solution with a 2-methylimidazole solution in a microreactor, and investigated the effect of the ratio of the concentration of 2-methylimidazole to that of zinc nitrate on the particle size. Our experiments demonstrated that the increase in the ratio decreases the ZIF-8 particle size, and the minimum particle size is 50 nm at the ratio of 200, which is accomplished by unequal flow rates of zinc nitrate and 2-methlimidazole. Further decrease in the particle size to 20 nm was achieved by the addition of n-butylamine molecules. We measured adsorption isotherms of N₂ in ZIF-8 particles and found that smaller ZIF-8 nanoparticles show the gate adsorption at higher pressures while the addition of n-butylamine decreases BET surface area.