Abstract: Natural Chinese cassia oil contains about 80% cinnamic aldehyde,and this major component is often available synthetically at a low cost.Cassia oil and synthetic cinnamic aldehydewere analyzed by GC/MC in order to detect synthetic cinnamic aldehyde added to cassia oil.Theresults show that phenyl pentadienal(PPDA),benzyl alcohol, eugenol and others are present insynthetic cinnamic aldehyde and are absent in natural cassia oil.The above mentioned impuritiescharacteristic of the synthetic preduct or raw materials can be detected by GC/MS.The lower limit ofdetection of PPDA and benzyl alcohol for this method corresponds to about 0.5%of added syntheticcinnamic aldehyde to cassia oil.