120-day winds of Sistan below in summer raging from middle of May to Middle of September, which affect large areas of Sistan and Baluchistan Province in southeast of Iran. They blow from northeast to southeast with high speed in summer and when the temperature reaches the highest value, the storm will be created. By changing the pressure pattern in winter, these winds will be vanished. Oman Sea and Strait of Hormuz (east of Persian Gulf) are directly influenced by 120-day winds of Sistan. The winds due to small scale are not in climatological and meteorological models, which results in lack of comprehensive studies in this field. The least-squire method using data of NCEP-NCAR with 2.5 ◦ spatial resolution was used to determine the period of 120-day winds of Sistan. The genetic factors of 120-day winds of Sistan show that wind is affected by the arrangement of pressure centers in the region. Genetic factors affecting 120-day winds included the relationship between pressure and heat in tropical and sub-tropical regions, establishing the low pressure center up to 700 hPa, approaching isobaric lines on the region, , making an inverse temperature gradient at the ground surface between Sistan region and south east of the Caspian Sea, getting active the low level vortex of troposphere, the effect of thermal factors, making pressure reverse slope and following the direction of the winds. 120-day winds blow from May to August so that significantly affect Oman Sea and Strait of Hormuz. Besides, the wind speed in east part of Oman Sea is more than that in west part of it. The highest value of 120-day wind speed is observed in July, and their 50% hazards influence the floating units with less than 500 tons in Oman Sea.