Microbial　aerobic　methane　oxidation　(MAMO)　affects　methane　emissions　through　landfill　covers　by　not　only　consuming　methane,　but　also　causing　biomass　accumulation　associated　with　bacterial　growth.　Although　the　reduction　of　soil　porosity　by　biomass　accumulation　has　been　well　recognized,　most　existing　models　ignore　this　effect　when　estimating　MAMO　efficiency.　The　present　study　proposes　a　newly　improved　theoretical　model　that　could　consider　the　effects　of　both　microbial　growth　and　biomass　accumulation　on　MAMO　during　coupled　water–gas–heat　reactive　transport　in　unsaturated　soil.　Comprehensive　batch　incubation　tests　were　performed　to　determine　the　input　parameters　required.　Part　of　a　set　of　published　experimental　data　was　used　to　validate　the　new　model,　while　the　remainder　of　the　dataset　was　used　to　evaluate　the　model　predictability　of　soil–microbe　interaction　(i.e.,　class　B　prediction).　When　ambient　temperature　is　relatively　high　(30　°C),　ignoring　biomass　accumulation　would　lead　to　an　overestimation　of　MAMO　efficiency　by　more　than　three　times.　As　the　biomass　accumulated　in　soil　pores,　the　water　permeability,　gas　permeability,　and　gas　diffusion　in　the　unsaturated　soil　reduced,　consequently　limiting　the　supply　of　oxygen　to　the　bacteria　for　MAMO　to　take　place.　©　2020,　Canadian　Science　Publishing.　All　rights　reserved.