Particle　size　is　an　important　parameter　to　determine　the　surface　modification　degree　of　sulfide　minerals　by　plasma　as　well　as　the　floatability　of　minerals.　However,　little　studies　have　been　reported　to　quantitatively　relate　surface　oxidation　and　sample　particle　size　to　flotation　performance.　In　this　study,　X-ray　photoelectron　spectroscopy　(XPS)　was　used　to　characterize　the　surface　species　of　arsenopyrite　and　pyrite　at　different　plasma　modification　times.　The　critical　oxidation　degree　was　quantified　as　the　proportion　of　hydrophilic　oxidation　species　to　hydrophobic　species　and　correlated　with　flotation　recovery.　The　results　showed　that　the　flotation　recoveries　of　minerals　with　different　particle　sizes　were　determined　by　surface　oxidation　rate　and　critical　oxidation　degree.　Fine　particles　were　more　likely　to　become　hydrophilic　under　low-temperature　plasma　modification　and　yet　the　critical　oxidation　degree　was　also　higher.　The　coarse　particles　being　plasma　modification　presented　a　poor　flotation　recovery　due　to　its　low　adsorption　density　of　collector,　although　its　oxidation　degree　was　lower　than　the　fine　size　fraction.　The　critical　oxidation　degree　of　pyrite　was　expected　to　be　less　than　arsenopyrite,　but　its　flotation　recovery　was　higher　under　the　same　plasma　medication　time　because　of　its　slower　oxidation　rate.　©　2022　The　Society　of　Powder　Technology　Japan