Among the diseases affecting leaf of tobacco, brown spot caused by Alternaria alternata is assuming importance in recent years in Northern region of Iran. The infected fields of Basma 178-2 variety in Minoodasht area (Golestan province) were sampled. After transfer of infected samples to laboratory and isolation of causal organism, 52 isolates were obtained. Considering morphological and biological characteristics, the causal organism was identified as A. alternata . These isolates were single spored and then were kept on Carrot Agar (CA) and Potato Dexterose Agar (PDA) at 25 °C. After determination the intensity of all the isolates considering number, size and kind of the brown spot in green house, K5 and 1mi from Minoodasht as the isolates with typical symptomes. The best temperature for fungus was 25 °C. Four fungicides were tested with control (water spray) on Basma 178-2 tobacco variety. The fungicides used were Propichonazol (Tilt), Tebuchonazol (Folicur), Ridomil MZ and Mancozeb. To determine the best fungicide for in vitro controling A. alternata , with 1, 10, 20, 30, 40, 50, 60, 70 and 80 PPM concentration for one week at 25 °C disk which were 5 mm in diameter from the active margin of the growing colonies were measured. In green house, Tilt and Folicur were tested with 0.5, 1 and 1.5 so Mancozeb and Ridomil MZ were tested with 1, 1.5 and 2/1000 concentration. In vitro and in vivo evaluation of fungicides show that Propichonazol (Tilt), Tebuchonazol (Folicur) and Mancozeb fungicides are found promising in the control of this disease 0.1, 0.1 and 0.2 percent concentration respectively.