Making　the　urban　buses　electric　is　regarded　as　a　major　strategy　to　reduce　greenhouse　gas　(GHG)　emissions　and　environmental　impacts　of　fossil　fuels.　In　practice,　not　all　diesel　buses　(DBs)　are　replaced　by　electric　buses　(EBs)　because　of　budget　constraint.　This　paper　investigates　the　balance　of　the　deployment　problem　for　a　mixed　fleet　with　DBs　and　EBs　in　the　sense　of　total　GHG　emissions　and　operational　costs　by　incorporating　the　effect　of　the　spatial-temporal　passenger　flows.　The　balance　strategy　of　fleet　deployment　is　defined　the　Pareto　optimal　allocation　of　EBs　among　bus　lines　to　minimize　simultaneously　the　total　operational　cost　and　GHG　emissions.　A　real-world　urban　bus　system　of　Liuzhou　City　in　China　is　conducted.　We　find　that　the　bus　lines　located　in　the　downtown　with　higher　passenger　loading　would　prefer　to　adopt　EBs　at　the　peak　hours,　and　most　DBs　are　allocated　to　the　bus　lines　with　long　travel　distance　at　off-peak　hours　in　the　suburb.　Therefore,　the　reduced　emission　by　adopting　EBs　mainly　concentrates　on　the　center　of　the　city,　and　more　produced　emissions　of　DBs　are　distributed　far　away　from　the　downtown.　When　all　DBs　replaced　by　EBs,　the　upper　bound　of　the　carbon　emission　reduction　ratio　is　77.04%,　which　reduces　from　207.15　tons　to　47.56　tons　per　day.