A　comprehensive　numerical　investigation　of　the　uncoupled　chemical,　thermal,　and　transport　effects　of　CO2　on　the　temperature　of　CH4/O-2　counterflow　diffusion　flame　under　high　pressure　up　to　5　atm　was　conducted.　Three　pairs　of　artificial　species　were　introduced　to　distinguish　the　chemical　effect,　thermal　effect,　and　the　transport　effect　of　CO2　on　the　flame　temperature.　The　numerical　results　showed　that　both　the　chemical　effect　and　the　thermal　effect　of　the　CO2　dilution　in　the　oxidizer　side　can　decrease　the　flame　temperature　significantly,　while　the　transport　effect　of　CO2　can　only　slightly　increase　the　flame　temperature　and　can　even　be　ignored.　The　reduction　value　of　the　temperature　caused　by　the　chemical　effect　of　CO2　grows　linearly,　while　that　caused　by　the　thermal　effect　increases　exponentially.　The　RPchem　and　RPthermal　are　defined　to　explain　the　temperature　reduction　percentage　due　to　the　chemical　effect　and　the　thermal　effect　of　CO2　in　the　total　temperature　reduction　caused　by　CO2　dilution,　respectively.　The　RPchem　decreases　with　the　increase　of　the　pressure,　the　strain　rate,　and　the　CO2　dilution　ratio,　while　the　RPthermal　thermal　behaves　in　the　opposite　manner.　In　the　above　conditions,　the　chemical　effect　plays　a　dominant　role　on　the　flame　temperature　reduction.