Objective To study the changes of volatile oil of Atractylodis Rhizoma(AR)and Atractylodis macrocephalae Rhizoma(AMR)before and after co-decocting in different proportions(1∶3,1∶2,1∶1,2∶1,3∶1).Methods The volatile oil was extracted by steam distillation and analyzed by gas chromatography-mass spectrometry(GC-MS).Results After co-decocting,the yield of volatile in each group was higher than in single decocting.The relative contents of most components in the mixed volatile oil after co-decocting were significantly different from the theoretical calculation.Co-decocting could promote the extraction of hinesol,which is the main component of the essential oil of AR,and the compatibility with large doses of AMR(1∶2 and 1∶3)could also promote the dissolution of β-eudesmol.At the same time,the extraction of atractylon,germacrene B and valencene was inhibited by co-decocting,which were the main constituents of the volatile oil of AMR.Conclusion Co-decocting of AR and AMR could improve the extraction rate and change the chemical composition of volatile oil.After co-decocting,the dissolution of active components of AR increased,while the dissolution of active components of AMR decreased.It may lead to the change of pharmacological activity.