Objective : To study the ultramicro decoction piece of Rhizoma Cyperi and establish its fingerprint. Methods : Capillary gas chromatography was used to analyze the extracts of ultramicro decoction piece of Rhizoma Cyperi from 10 different sources. Results: The Fingerprint of ultramicro decoction piece of Rhizoma Cyperi was composed of 50 peaks, containing 10 characteristic peaks. Conclusion: The fingerpint can be used to control the ultramicr...