The　catalytic　conversion　of　methane　into　value-added　products　under　mild　reaction　conditions　presents　a　key　challenge　in　heterogeneous　catalysis.　In　this　study,　we　investigate　the　conversion　of　methane　to　acetic　acid　(CH3COOH)　and　other　methyl　oxygenates　by　using　molecule　CO　as　the　promoter　on　Ir/ZSM-5　catalyst.　An　optimum　space-time　yield　of　7.4　mmol　center　dot　gcat-　1　center　dot　h-　1　to　acetic　acid　was　achieved　under　mild　conditions　(2.5　MPa　CH4,　2.5　MPa　CO,　80　degrees　C).　The　influence　of　CO　on　methane　conversion　was　investigated　across　various　transition　metals.　Ir,　Fe,　Rh,　and　Pd　showed　promotion　towards　oxygenated　compound　formation,　while　Cu　and　Cr　exhibited　inhibition.　Sequential　charging　of　CH4　and　CO　experiments　reveal　the　critical　role　of　CO　in　acetic　acid　production.　Furthermore,　the　role　of　CO　was　further　investigated　by　in　situ　Raman,　in-situ　FTIR　and　EPR.　This　work　shows　the　role　of　CO　in　mild　conversion　of　CH4　to　CH3COOH　on　Ir　catalyst.