In our results, if explained by WHO guideline and applying diagnosis standard 1997, DMFT index is a non-reversible index because DT or DS (average of carious teeth or carious tooth surface) can not be annulled but transformed into MT, MS (average of teeth or tooth surfaces missed by caries), or FT, FS (filled teeth and tooth surfaces). Thus, DMFT and DMFS are accumulated by time and non-reversible.