Thermal environment of the mud is one of the major parts of the acid-contaminated geo-environment of the Ariake Sea. The activities of marine ecosystem both in tidal flats and inside the deep sea mud depend strongly on the thermal environment. In order to investigate the thermal properties of the Ariake sea mud, a portable thermal properties sensor and probe (KD2, Decagon Devices, Inc.) was used. Thermal conductivity, thermal resistivity, thermal diffusivity and volumetric heat capacity of the mud samples collected from both the tidal flat and inside the deep sea were measured. The thermal properties of mud collected from tidal flat showed a different trend from the mud collected from inside the sea due to the enough exposure to the sunlight and vigorous exchange of sea water in the tidal flat in different depths. Thermal conductivity of the Ariake mud was reduced 85-90% after it was oven dried and was increased up to 30% after the ignition loss test. Thermal resistivity of the Ariake mud increased dramatically after oven dried and it was less affected by the organic matter content. Thermal diffusivity was decreased 10-30% after oven dry however it was increased up to 40% after the ignition loss test. The volumetric heat capacity of the Ariake mud was reduced up to 90% and 40% after the oven dry and ignition loss, respectively.