Crystal size and morphology of zeolitic imidazolate frameworks (ZIFs) can be generally controlled based on the classical theory of nucleation and growth. Herein, we have developed an alternative method to adjust the nucleation and growth kinetics of microporous ZIF-8 nanocrystals mediated by continuous CO2 gas bubbling. In particular, CO2 bubbling led to the dissolution of ZIF-8 slurry, while the evacuation of CO2 bubbling resulted in the formation of new ZIF-8 nanoparticles with a considerably smaller size. A plausible mechanism of the CO2-mediated synthesis of ZIF-8 nanoparticles was proposed based on comprehensive characterizations and analyses, which indicated that the dissolved CO2 in methanol was able to perturb the pre-equilibrium states of crystallization intermediates and led to a comparatively fast nucleation rate due to a low number of overcoordinated species between the metal ion and the ligand. Both methanol and the base were critically important to the dissolution-recrystallization of ZIF-8, wherein the methyl carbonate linker might be reversibly produced by CO2 insertion into the methoxide group (Zn-OCH3). Also, the CO2-mediated synthesis led to the small particle size, high crystallinity, good thermal stability, and high purity of ZIF-8, as compared to the conventional ZIF-8 prepared without CO2 gas bubbling. As proof of workability, the prepared monodispersed ZIF-8 nanoparticles showed a much higher photocatalytic activity toward various organic dyes' decomposition than the conventional ZIF-8. Also, the CO2 bubbling-mediated method could be further extended to prepare other ZIFs (e.g., ZIF-67).