AbstractZr‐based metal‐organic framework (Zr‐MOF) nanoparticles (NPs) have attracted extensive research thanks to their outstanding thermal and chemical stability, but their efficient synthesis is still challenging. Here, high‐quality Zr‐MOF NPs with tunable and uniform particle sizes can be successfully fabricated within 30 min by high‐gravity technology with high yields. Significantly, the rapidly milder preparation of UiO‐66 NPs can also be achieved at 90 or 70°C. This strategy has been extended to the synthesis of other Zr‐MOF NPs. Furthermore, the water vapor adsorption properties of obtained UiO‐66 NPs display a size‐dependent effect. The 70 nm UiO‐66 NPs have the highest adsorption capacity of 625 mg g−1 among unmodified UiO‐66 reported so far, and manifest 2.0–2.8 times faster water adsorption rate than the micron ones. This study provides a feasible method for the efficient and mild preparation of MOFs nanoparticles, which may promote the synthesis and applications of nano‐sized MOFs.