Evapotranspiration is one of the critical parameters in the planning and management of water resources, especially in the agricultural sector. In this study have been discussed to review the reference evapotranspiration from the perspective of main factors its aggravating, with an emphasis on pressure systems in the warm season over the period 2014 1985 in the South West of the country. For this purpose at first, reference evapotranspiration calculated and then correlation values with the parameters of temperature, radiation, humidity and wind were calculated and finally to review of pressure systems, the average data of sea level pressure, zonal and meridional wind from the June to August in the period of 1985 - 2014 provided and maps were drawn. The results show that the correlation maximum of the reference evapotranspiration with wind parameter obtained So that in more than half of the studied stations, correlation coefficient has been more than 0.90 and almost at all stations (except station in Bushehr and coastal Bushehr) was more than 0.80. Among the parameters, moisture in many stations has had negative correlation with evapotranspiration. The radiation parameter than temperature parameter the more positive correlation has experienced in most stations. Review of pressure systems summer at sea level pressure indicates that the interaction of the western tongues monsoon low pressure system and eastern tongues Azores subtropical high pressure on the East Mediterranean sea is as the main factor of pressure gradient on the Middle East, including the South West of the country. That resulted northwest prevailing wind flow from the eastern Mediterranean conduct to the study area. As well as the prevailing wind flow in the study area from June to August, has been the Northwest. Cause of Strengthen and increase the wind speed and the result has been an increase in evapotranspiration, which would be a serious risk, especially in enhancing crop water requirement is due to the limited available water.