Abstract: According to the daily synoptic charts of April to September for the period 1976-1986, the fundmental characters, the formation and the ways by which the effects can take place on the weather of the western part of south China of the midtropospheric cyclones over the area between 10°N and 30°N,100°E and 130°E are studied in this paper.Moreover,based on the data for 07h of Monthly Aerological Bulletin of China for the same period,fifteen influential midtropospheric cyclones whose centers are in south China or the offshore area are selected for studying.Expecially,the structure of the midtropospherie cyclones which have an effect on the western part of south China are investigated systematically by composite method.Finally,the key points to determine whether the midtroposphric cyclones influence the western part of south China are proposed.