The　gas　diffusion　(Dg)　and　permeability　(Kg)　coefficients　are　the　two　important　parameters　for　the　study　on　gas　transport　in　unsaturated　soils.　In　the　present　study,　the　theory　of　calculating　Dg　and　Kg　for　unsaturated　soils　under　steady　state　is　derived.　Dg　and　Kg　of　unsaturated　soils　are　determined　based　on　the　relationship　between　tracer　gas　concentration　(C)　and　gas　flow　rate　(q1)　and　that　between　pressure　gradient　(p)　and　gas　flow　rate　(q2),　respectively.　A　device　for　in-situ　measurement　of　Dg　and　Kg　in　unsaturated　soils　is　designed　and　developed.　The　soil　column　tests　are　conducted　to　measure　Dg　and　Kg　of　unsaturated　soils　with　different　water　contents.　Thereafter,　the　element　tests　and　numerical　simulation　analysis　are　used　to　verify　the　effectiveness　of　the　new　method　and　device.　It　is　found　that　Dg　and　Kg　obtained　by　the　new　method　are　in　good　agreement　with　the　results　of　the　element　tests　and　numerical　simulations.　The　results　of　the　sensitivity　analysis　show　that　the　measurement　accuracy　of　Dg　reduces　as　the　soil　water　content　decreases,　while　the　measurement　accuracy　of　Kg　is　independent　of　water　content　of　soil.　The　measurement　accuracies　of　both　Dg　and　Kg　decrease　with　the　increase　of　the　radius　of　the　aeration　bulb,　but　the　measurement　of　Kg　is　more　sensitive　to　the　change　of　the　radius.　The　measurement　accuracy　of　Kg　is　about　55%　of　the　true　value,　when　the　radius　of　the　aeration　bulb　is　0.05　m.　The　soil　anisotropy　reduces　the　measurement　accuracy;　when　the　values　of　Dgv/Dgh　and　Kgv/Kgh　is　0.1,　the　measurement　accuracies　of　Dg　and　Kg　can　still　reach　more　than　40%.　©　2024　Chinese　Society　of　Civil　Engineering.　All　rights　reserved.