Regarding chemical changes in oriental drugs after food processing such as roasting, fermentation, and extrusion, fifty commonly-used medicinal plants were investigated. As a result, Aurantii nobilis Pericarpium (a tangerine peel from Citrus unshu Markovich) showed remarkably different HPLC profiles after being roasted. An increased peak was isolated by repeated chromatography and identified as 5-hydroxymethyl furfral (5-HMF) by means of instrumental analyses. The 5-HMF content of Aurantii nobilis Pericarpoum reached its maximum level after being roasted for 30 min at 225$^{circ}C$ (49.2 mg/g extract, ca 42 times of increase over untreated control). Although there were no significant changes in in vitro biological activity such as antioxidative, anti-dementia, anti-hypertension, anti-coagulation, or cytotoxicity, before and after roasting process, our results suggested that simple heat treatment might improve the value of the above oriental drug since 5-HMF has been known to possess inhibitory activities toward nitric oxide formation, tyrosinase, and sickling of red blood cells.