Gas chromatography/mass spectrometry, chemometric resolution method-Alternative movingwindow factor analysis that were proposed recently and the Kovats retention index were used to analyze the essential components of herbal pair Cinnamomum cassia Presl and Poria cocos (CCP-PC) and compare them with those of single herbal Cinnamomum cassia Presl (CCP). 46 and 42 essential components in essential oil of CCP and CCP-PC have been identified individually. Results shows that the number of essential components of CCP and CCP-PC were almost the same, but extractive ratioes of them have changed significantly, some of them were increased obviously, most of them were declined notably instead and even several ingredients of CCP-PC were disappeared due to CCP PC's interaction probably. The main pharmacodynamic ingredients of CCP-PC, 3-phenyl-2-propenal and Cinnamaldehyde were obviously higher contents than that of single CCP. It suggested that there exist certain interactions of the chemical ingredients in compound medicine rather than their sum effect of single medicines. There is a litter difference in quality and quantity between single medicines and pair medicines, so the application of pair medicines can expand single medicine's adaptive disease and has a good clinical referenced valuation.