Nowadays, diabetes mellitus (D.M) is one of the most common diseases in the world and the rate of afflicted persons is increasing each year. D.M causes metabolic disorders with high mortality rate. Although this health problem is important , but accomplished researches are relatively rare in Iran. Present study has been done to study of epidemiological factors of diagnosed D.M. in Arak. The study showed that the rate of prevalence of diagnosed D.M. is about 7% and it is lower than the other performed studies. Among 18637 people (9622 M.& 9015 F.) persons (61%) were Non-Insullin dependent diabetes mellitus (N.I.D.D.M) and 14 persons 8% (0.08) were insulin dependent D.M.(I.D.D.M). It means that the disease is unknown by the people, and more studies should be planned in field of screening of D.M. in Arak.