Nanocrystalline zeolitic imidazolate frameworks 8 and 67 (ZIF-8 and ZIF-67) were prepared via surfactant-assisted synthesis in aqueous solution at room temperature. The synthesis without surfactant yielded crystal sizes for ZIF-8 and ZIF-67 of 1.19 and 2.95 mu m, respectively. However, their crystal sizes obtained via surfactant-assisted synthesis were 81 and 150 nm, respectively. The Brunauer-Emmett-Teller (BET) surface area and micropore volume remained almost constant regardless of the crystal size. TGA analysis confirmed that ZIF-8 and ZIF-67 have sufficient thermal resistance for water treatment. Highly crystalline ZIF-8 and ZIF-67 with different crystal sizes could be obtained, and these ZIF crystals were used as adsorbents for adsorptive removal of perfluorooctane sulfonate (PFOS) from aqueous solution. The amount of PFOS adsorbed by ZIF-67 (734.7 mg-PFOS/g-ZIF-67) was considerably higher than that absorbed by zeolite, activated carbon, and ZIF-8. Additionally, the crystal downsizing was effective in improving the adsorption rate, and adsorption equilibrium was reached faster by nanocrystalline ZIF-67 than by macrocrystalline ZIF-67. This feature makes nanocrystalline ZIF-67 one of the most effective and promising adsorbents for removing PFOS from aqueous solution.