Secondary　natural　gas　supply　was　used　to　study　emissions　such　as　NOx,　CO　and　unburned　hydrocarbon　(UHC)　and　the　temperature　at　the　outlet　of　catalytic　monolith　as　well.　The　suitable　proportion　of　first　fuel　supply　to　the　secondary　was　found　for　lower　combustion　emissions.　The　experimental　results　showed　that　the　appropriate　secondary　fuel　supply　helped　to　reduce　NOx　emission,　while　CO　and　UHC　emissions　still　remained　low.　However,　with　the　increase　of　secondary　fuel　supply,　though　NOx　emission　almost　kept　uncharged,　CO　and　UHC　emissions　constantly　increased　when　the　secondary　fuel　volume　was　more　than　a　certain　amount.　Meanwhile,　the　experiments　also　showed　that　the　secondary　fuel　supply　reduced　the　temperature　in　the　catalytic　monolith,　which　helped　to　protect　the　catalytic　monolith　from　fracture　caused　by　high　heat　stress　and　from　deactivation　of　catalyst　caused　by　high　temperature.