This　study　developed　a　new　method　to　simultaneously　measure　gas　dispersion　coefficient　and　gas　coefficient　of　permeability　(GCP)　of　an　unsaturated　soils　with　due　theoretical　consideration　of　steady-state　gas　transport　mechanisms.　Correspondingly,　a　soil　column　apparatus　was　developed.　Based　on　the　proposed　method,　gas　mixture　containing　air　(with　a　volumetric　concentration　＞90%)　and　tracer　gas　were　applied　to　the　column　base.　The　gas　dispersion　coefficient　was　estimated　based　on　a　measured　steady-state　tracer　gas　concentration　gradient　and　a　dispersive　flux　of　the　tracer　gas,　the　latter　of　which　was　determined　based　on　mass　conservation　of　the　tracer　gas.　On　the　other　hand,　the　GCP　was　determined　based　on　the　total　flux　of　gas　mixture　and　gas　pressure　gradient　following　Darcy＇s　law.　The　new　method　has　the　following　advantages　over　existing　methods:　(1)　has　an　ability　to　determine　gas　dispersion　coefficient　and　GCP　at　any　soil　depth　rather　than　an　average　value　of　a　test　region;　(2)　measures　the　two　transport　properties　at　varying　soil　moisture　conditions　rather　than　being　limited　by　uniform　moisture　distribution;　and　(3)　requires　only　a　simple　hand-calculation　method　to　estimate　the　gas　dispersion　coefficient　without　the　need　to　apply　nonlinear　regression　on　a　gas　breakthrough　curve.　The　proposed　method　to　determine　GCP　and　gas　dispersion　coefficient　of　a　silty　sand　was　verified　by　independent　element　tests.(c)　2022　American　Society　of　Civil　Engineers.