The catalytic activity of supported noble metal nanoparticles (NPs) is strongly dependent on their size. However, it is still a pronounced challenge to develop a facile method to reduce the size of noble metal NPs. Here we report a facile but efficient strategy by utilizing confined spaces between templates and silica walls in as-prepared SBA-15 (AS) to regulate the size of a well-known noble metal, rhodium (Rh). Rh NPs formed in confined spaces possess obviously smaller size than their counterparts generated in traditional calcined SBA-15 (CS) without confined spaces. Our results also demonstrate that the obtained materials are highly active in CO oxidation. For a typical material 5.0RhAS containing 5.02 wt % of Rh, the complete conversion of CO is achieved at 70 degrees C, which is evidently lower than that of its analogue 5.0RhCS (102 degrees C).