Developing　new　synthetic　methods　for　carbon　supported　catalysts　with　improved　performance　is　of　fundamental　importance　in　advancing　proton　exchange　membrane　fuel　cell　(PEMFC)　technology.　Continuous-flow,　microfluidic　reactions　in　capillary　tube　reactors　are　described,　which　are　capable　of　synthesizing　surfactant-free,　ultrafine　PtSn　alloyed　nanoparticles　(NPs)　on　various　carbon　supports　(for　example,　commercial　carbon　black　particles,　carbon　nanotubes,　and　graphene　sheets).　The　PtSn　NPs　are　highly　crystalline　with　sizes　smaller　than　2nm,　and　they　are　highly　dispersed　on　the　carbon　supports　with　high　loadings　up　to　33wt%.　These　characteristics　make　the　as-synthesized　carbon-supported　PtSn　NPs　more　efficient　than　state　of　the　art　commercial　Pt/C　catalysts　applied　to　the　ethanol　oxidation　reaction　(EOR).　Significantly　enhanced　mass　catalytic　activity　(two-times　that　of　Pt/C)　and　improved　stability　are　obtained.