The　polyhedral　Pd　and　Pt　nanocrystals　with　well-defined　morphologies　were　synthesized　via　chemical　reduction　method　in　an　aqueous　or　polyol　system.　The　catalytic　CO　oxidation　activities　over　the　catalysts　with　the　polyhedral　Pd　and　Pt　nanocrystals　supported　on　SiO2　were　evaluated.　It　was　found　that　the　CO　oxidation　activity　over　the　Pd-O/SiO2　catalyst　was　higher　than　that　over　the　Pd-C/SiO2　catalyst,　and　the　dominative　exposed　Pd　nanocrystal　facet　was　(111)　for　the　former　and　(100)　for　the　later　catalyst.　The　Pt-C/SiO2　catalyst　loaded　with　Pt　cube　nanocrystals　mainly　exposing　(100)　facets　showed　higher　CO　oxidation　activity　than　the　Pt-O/SiO2　catalysts　with　the　(111)　plane　as　the　mainly　exposed　facet.　The　investigation　revealed　that　the　Pd(111)　and　Pt(100)　facets　possessed　appropriate　CO　adsorption　strength　that　was　responsible　for　the　high　activity　of　CO　oxidation,　whereas　CO　adsorption　over　the　Pt(111)　was　so　strong　that　the　CO　self-poisoning　occurred　over　the　Pt-O/SiO2　catalyst.　The　CO　adsorption　on　the　Pd(100)　was　so　weak　that　the　CO　molecules　could　not　be　activated　over　the　Pd-C/SiO2　sample.　Thus,　the　CO　oxidation　over　the　Pt-O/SiO2　and　Pd-C/SiO2　catalysts　was　suppressed.　(C)　2012　Elsevier　B.　V.　All　rights　reserved.