Abstract: OBJECTIVE To identify and analyze the chemical composition of the volatile components in Baizhu Shaoyao San (BZSYS)， decoction sample composed with unprocessed herbal drugs was tested for a purpose of comparation. METHODS The volatile oil was extracted from BZSYS decoction by steam distillation， diluted and then separated by GC-MS. Components were matched and identified based on NIST 11 and Wiley databases. The relative contents of these components were calculated by normalization method. Finally， the volatile oil samples obtained from processed or unprocessed drugs were compared and analyzed. RESULTS A total of 145 components were matched by data analysis in both groups. After processing， 15 components were found to be disappeared， while 17 new components were found to be appeared. CONCLUSION The processing of herbal drugs shows a significant influence on the volatile components of BZSYS. It is a realistic and objective reason that BZSYS should be composed with processed drugs just as recorded in the ancient literature.