Noble-metal nanoparticles have attracted great attention because of their excellent catalytic activity. However, the activity of noble metal nanoparticles is severely dependent on the sire of the metal nanoparticles. Therefore, regulation of the nanoparticle size is of great importance. Herein we report an efficient strategy for the first time to regulate the size of platinum (Pt) nanoparticles in atypical mesoporous silica SBA-15 by using the confined spaces. The Pt-containing precursor is -introduced to the confined spaces between the template and silica walls in as-prepared SBA-15 by grinding. Subsequent calcination allows template removal and precursor conversion in one step and avoids repeated calcination in conventional modification processes.. This leads to the formation of Pt nanoparticles with smaller-size, making Pt metal catalysts highly active. Hence, the catalytic activity of the. oxidation of trace ethylene is very superior to that of those prepared by the conventional method. For a typical material, PtAS-5, containing 5.0 wt % Pt, the complete conversion of ethylene was achieved at 40 degrees C, which is lower than that on the catalyst PtCS-5 prepared, by the conventional method (50 degrees C) as well as that on a series of catalysts reported previously.