A new spray-drying process of nanoparticle-dispersed aqueous solutions was proposed to produce metal/metal oxide fine-particle powder with controlled particle aggregation, in which high-density carbon dioxide is used as both an aqueous colloid dispersing solvent and a compressible fluid for spraying. An experimental setup was built based on the proposed idea and verified with aqueous solutions of 8-nm zirconia particles. The aqueous colloidal content was set to be more than 10 wt% from the viewpoint of practical production. This is an extremely high content in the high-density CO2 phase from the viewpoint of thermodynamic stability and leads to colloid aggregation within a short time, followed by phase separation. The proposed idea focuses on the time scale of such unstable colloids and requires the colloids in CO2 media to be sprayed via a nozzle within the short time before colloid aggregation begins. In addition, the sprayed colloids could be broken down into small droplets and frozen due to Joule-Thomson expansion of high-density CO2. The results showed that powders made of nanoparticles contained in frozen droplets could be generated with controlled particle aggregation.