Abstract: The chicken and duck eggshell powders were prepared and heated at 1300°C for 4 h. The samples were analyzed by X-Ray Diffraction (XRD). It was found that Calcium Carbonate (CaCO3) from chicken and duck eggshells completely transformed into Calcium Oxide (CaO). Then the samples were placed in a box installed with a hygro-thermometer for humidity adsorption test in the range of 20-75% RH at 25°C. Hydration rate for duck and chicken eggshells were 0.0625% RH min-1 (within 880 min) and 0.0482% RH min-1 (within 1140 min) respectively. The results show that CaO from the duck eggshells is a better adsorbent than that of chicken eggshells. Furthermore, after humidity adsorption, CaO obtained from both eggshells were partly transformed into portlandite (Ca(OH)2 was confirmed by the Rietveld refinement method and Scanning Electron Microscope (SEM).