Germinated wheat and barley were extruded with a pilot-scale corotating twin-screw extruder under different barrel temperatures (100 and 120 °C), feed moisture content (30 and 35%) and CO2 injection. The effect of germination, barrel temperature, feed moisture content and CO2 injection on physical properties, proximate composition, γ-aminobutyric acid, β-glucan, phytic acid and protease activity was investigated. Results indicated that the barrel temperature, moisture content and CO2 injection significantly affected water absorption index (WAI), water-soluble solid index (WSI), specific length, expansion ratio, bulk density, mechanical properties and colour of all extruded products (P < 0.05). Extrusion process significantly decreased γ-aminobutyric acid and protease activity in extruded germinated wheat (EGW) and barley (EGB) products (with and without CO2 injection). However, extrusion process (with and without CO2 injection) increased β-glucan in EGB and decreased phytic acid in EGW. The content of proximate composition, γ-aminobutyric acid, β-glucan, phytic acid and protease activity was slightly affected by CO2 injection.