Abstract: The volatile constituents in the fruits and leaves of Eucalyptus citriodora,were extracted by using steam distillation method,the components of oil were analyzed by gas chromatography- mass spectrometry( GC- MS)and to find the difference. Studied the effect of the main components of the fruits of Eucalyptus citriodora The results show that,the yield of fruits oil was 0.76 m L /100 g( on fresh weight basis),leaves was 1.3 m L /100 g. The result of GC- MS showed that 37 component were identified in the fruits oil,Composed mainly of terpenes,there were 9 monoterpene,8 oxygenated monoterpenes,5 sesquiterpene,7 oxygenated sesquiterpene,7 others compound,1 aromatic compound. The major constituents in the fruits oil of Eucalyptus citriodora were 1R- α-Pinene,followed by γ- terpinen、caryophyllene and so on. the main components of Eucalyptus oil were citronellal,citronellol,isopulegol. Both components less the same: α- pinene,isopulegol,α- terpineol,etc.,there are big differences in the content.