According to the famous ancient Greek traveller Pausanias, the ancient Greek theatre of Megalopolis was the biggest in the Greek territories. It had a diameter of 126 metres and a capacity of 21.000 persons and it probably accommodated the meeting of the Common of the Arcadians. The architectural parts of the theatre are built with whitish, sedimentary limestone, while the surrounding walls of the edifice to the east and west are made of stone. Agents such as water, atmospheric conditions and the microclimate have caused alterations in the building materials, changes in colour, composition, natural properties and so on. The conservation practices proposed are cleaning of depositions, salt removal from stone surface, treatment of the theatre’s foundations to make them waterproof and durable. In addition consolidation of building materials is proposed, as well as restoration of broken architectural parts, reconstruction of surrounding walls and compulsory use of suitable filters by the factories of Megalopolis.