Objective To investigate the effects of grinding size and extraction temperature on the chemical composition, colloidal properties, and antioxidant activity of coffee colloidal particles, and to elucidate their formation and characteristics.Methods Coffee infusions were prepared from whole beans, medium-ground (0.9 mm), and fine-ground (0.3 mm) coffee beans using hot brewing (100 ℃, 3 min) and cold brewing (4 ℃, 24 h). The content of total solids, total sugars, proteins, total polyphenols, and lipids were determined. The colloidal properties were characterized using a nanoparticle size and zeta potential analyzer. The infusions were separated into a filtrate (true solution phase) and a retentate (colloidal phase) using an ultrafiltration membrane with a nominal molecular weight cut-off of 1×10⁵. The chemical composition and antioxidant activity of each phase were analyzed separately. Extracellular antioxidant activity was evaluated using ORAC, ABTS, and DPPH assays, while intracellular antioxidant activity was assessed using the CAA assay.Results Fine grinding (0.3 mm) combined with hot brewing significantly increased the dissolution rates of total solids, polyphenols, and lipids (P<0.05) and promoted the formation of colloidal particles (light scattering intensity of hot brew: 115 501.8 kcps, far exceeding 1 250.2 kcps for cold brew). Ultrafiltration showed that lipids and other components were enriched in the colloidal particles. The true solution phase (filtrate) primarily contributed to extracellular antioxidant activity, while the colloidal phase (retentate) exhibited significant biological activity in the CAA assay.Conclusion High-temperature extraction and fine grinding significantly enhance the formation of colloidal particles by promoting the dissolution and self-assembly of components. These colloidal particles constitute the key material basis for the intracellular antioxidant activity of coffee.