Abstract: The yield and composition of essential oils were studied.They were obtained from the withered balde tip,fresh stem,fresh whole grass and dry whole grass of harvested Cymbopogon citratus (D.C.) Stapf produced in Fangchenggang City,Guangxi by steam distillation.The effects of mildewing on the essential oil were investigated.The results showed that the C.citratus decayed along with time in hygrothermal environment.According to the experimental results,it was seen that not only the principal components in essential oils,such as neral and geranial,but also the yield from grass was reduced.The oil yield of fresh whole grass and dry whole grass were almost equal.The yield and citral content were 0.483% and 71% from fresh C.citratus essential oil,but they reduced to 0.317% and 66.1% from withered grass when they was decayed after 7 days.The highest content of citral with 78.8% was obtained from the fresh stem essential oil, and the lowest oil yield with 0.17% was obtained from the withered balde tips.