According to Pausanias, the ancient theatre of Megalopolis was the largest one in the Hellenic area, having 126 m diameter and a capacity of 21.000 persons. It was probably used for the meetings of Arcadian Civil Commission. All architectural parts of the theatre are from whitish sedimentary limestone, while both the east and west surrounding walls are from conglomerate stone. The samples for the laboratory study and analysis were taken from five different representative areas of the monument. A second series of samples were collected from the area around the theatre and from a local quarry, which was possibly the source for the stones used in the theatre. Based on bibliographic review and analytical data, some valuable conclusions about the provenance of stones occurred. Deterioration agents of the area, such as water, polluted atmosphere and microclimate conditions have resulted alterations to the building materials of the monument (changes in colour, composition, structure, physical properties etc.). Proposed conservation practice, both preventive and active, contains cleaning of depositions using absorbent clays and biocides, desalination of stone surface using neutral paper and distilled water and suitable treatment of foundation focused on the removal of humidity and the limitation of the capillary rise phenomenon. Additionally, consolidation of the building material is suggested using saturated solution of calcium hydroxide, silicate esters and silicate resins, as well as getting the broken parts together using titanium joints and mortars compatible to the original materials, completion of lost parts with a limestone similar to the original, reconstruction of the surroundings walls and application of suitable filters in the chimneys of Megalopolis factories of Electricity for the elimination of the released pollutants.