Insightful　understanding　of　the　reaction　mechanisms　that　control　both　growth　and　phase　formation　is　one　of　the　most　challenging　issues　in　ordered　intermetallic　catalysts.　In　this　study,　we　developed　a　microfluidic　system　with　a　millisecond　time　resolution　for　the　synthesis　of　PtSn　intermetallic　nanocrystals,　with　which　the　complex　formation　mechanism　of　intermetallic　phase　was　enabled　to　investigate.　The　result　revealed　that　the　formation　of　PtSn　nanocrystals　occurs　in　four　stages:　increasing　monomer,　burst　nucleation　of　monomers　and　intermetallic　phase　formation,　growth　and　particle　attachment,　and　sintering　stage.　The　products　in　the　range　of　milliseconds　to　seconds　can　be　obtained　by　different　lengths　of　reaction　microchannels.　This　setup　allowed　control　of　the　phases　of　the　PtSn　nanocrystal　growth　and　thereby　tuned　the　catalytic　performance　of　the　ethanol　oxidation　reaction.　Such　an　understanding　of　the　growth　of　pathways　is　significant　for　the　design　of　ordered　intermetallic　nanocatalysts.