Highly active CaO catalyst was prepared by the calcination–hydration–dehydration treatment of egg shells. Egg shells were washed thoroughly in tap water to remove any unwanted material adhered on its surface, and rinsed twice with distilled water. The washed egg shells were then dried in hot air oven at 105 C for 24 h. The dried egg shells were reduced to small pieces and calcined in a muffle furnace under static air conditions at 900 C for 2.5 h to transform the calcium species in the shell into CaO particle (denoted as Egg shell-CaO-900). Then the CaO derived from the egg shell was refluxed in water at 60 C for 6 h and the solid particle was filtered and dried in hot air oven at 120 C overnight. The solid product was dehydrated by performing calcination at 600 C for 3 h to change the hydroxide form to oxide form. Thus the egg shells subjected to the calcination–hydration–dehydration treatment generate a highly active CaO (denoted as Egg shell-CaO-900-600).