Adsorption and thermodynamic properties of Ar on ZnO powder were studied using a newly built gas adsorption isotherm apparatus, which has good temperature stability within 0.01 K in a wide rage (20-300 K) and good pressure accuracy with a resolution better than 10 −5 in 100 torr. Two distinct Ar atomic layers were measured. A specific surface area of the ZnO particle was also obtained from calculations of a molecular area of Ar on the surface of the adsorbents, and found to be approximately 100 m 2 /g. The thermodynamic results including the 2-dimensional compressibility indicate the surface melting occurs near 77 K.