Pure molybdenum trioxide was produced from the roasted molybdenite concentrate of Sarcheshmeh mines " Kerman, south east of Iran ". The roasted products were dissolved in an ammonia solution and the liquor was purified with 20% (vo%l)ammonium hydrogen sulfide after which it was mixed with 2M hydrochloric acid to precipitate molybdic acid. Molybdic acid was dissolved in 2.37N-ammonium solution from which ammonium paramolybdate was crystallized and precipitated. Ammonium paramolybdate was calcined in a furnace in the temperature range 300-400 OC to produce pure molybdenum trioxide. The effect of different parameters such as temperature, time, solid-liquid ratio in leaching, pH, concentration and amount of ammonium solution on production of the desired products were studied in different stages and optimum conditions for production of pure MoO3 were determined