Regarding chemical and biological activity changes in oriental drugs after food processing such as roasting, fermentation, and extrusion, fifty commonly-used medicinal plants were investigated. As a result, cinnamon (Cinnamomi Cortex) showed remarkably different HPLC profiles and ca 40% of increased antioxidative effect after being roasted. An increased peak was isolated by a reverse phase silica gel chromatography and identified as cinnamaldehyde by means of instrumental analysis including $^1H$ and $^{13}C$ NMR and MS. The enhanced antioxidative effect might originate from the increased content of cinnamaldehyde since this compound is one of the most well known natural antioxidants. The cinnamaldehyde content in cinnamon reached its maximum level after being roasted for 10 min at $120pm2^{circ}C$ ($108.42pm0.26$ mg/g extract, ca 137 times of increase over untreated control). Although there were no significant changes in in vitro biological activity such as anti-dementia, anti-hypertension or cytotoxicity, before and after the roasting process, the results suggested that simple heat treatment might improve the value of the above oriental drugs since cinnamaldehyde has been shown to possess various biological properties such as antibiotic, antioxidative, anti-diabetic, anti-influenza, and apoptosis-inducing properties.