Atractylodis rhizome is an important Chinese medicinal herb which is used extensively in the practice of Traditional Chinese medicine (TCM). Processing is a common practice and usually occurs before Atractylodis rhizome is prescribed. However, how to evaluate the quality of processed Atractylodis rhizome has not been reported. In order to evaluate the composition of essential oil induced by the processing, the essential oil from the crude and processed Atractylodis rhizome (Mao-Cangzhu (geoherb) and Ying-Cangzhu (non-geoherb) was isolated by hydrodistillation and distillation-extraction. Chemical constituents of the essential oil were separated and identii¬ed by means of gas chromatography-mass spectrometry (GC-MS), and the relative content of each constituent was determined by internal standard method. The results showed that the essential oil between Mao-Cangzhu and Ying-Cangzhu was markedly quantitatively and qualitatively different. Processing resulted in the reduction of essential oil content and variation of chemical composition in Atractylodis rhizome. Hierarchical cluster analysis showed that processing led to the chemical composition of the essential oil in non-geoherb to be more close to that of geoherb. It is feasible to establish standards for evaluating the quality of processed Atractylodis rhizome considering characteristics of essential oil in geoherb. Key words: Atractylodis rhizome, essential oil, processing, crude, geoherb.