Using HPLC, we examined quantitative change of cinnamaldehyde and cinnamic acid, caused by time-progress and temperature-change for the Cinnamoni Cortex preparations, Gye Ji Ryung Whan(桂枝茯?丸), Pal Mi Ji Whang Whan(八味地黃丸) and Do Haig Sung Ki Tang(桃核承氣湯). The content of cinnamaldehyde significantly decreased by the increase of temperature. It is suggested that cinnamaldehyde and cinnamic acid are considerably stable when kept frozen.