Objective To develop a high-performance liquid chromatography-diode array detector (HPLC-DAD) method for determination of cinnamic acid in the decoction of cinnamon, and investigate the effect of different storage temperature and time for the stability of cinnamic acid. Methods An HPLC- DAD method was established. Separation was performed on an Agilent Zorbax C₁₈ column (4.6 mm×250 mm, 5 μm) with 0.1% formic-acetonitrile acid water solution (60:40) as the mobile phase by isocratic elution. The flow rate was 1.0 ml/min, the temperature of column was 25 ℃, the injection volume was 5 μl, the detective UV wave length was 275 nm. The decoction were stored under refrigerated temperature (4 ℃) ambient temperature (25 ℃) and high temperature (40 ℃). The cinnamic acid was detected after 0, 1, 3, 7, 14, 21, 30 d. Results Cinnamic acid was successfully separated by this method, with good linear relationship between 10.21-204.20 μg/ml. The precision, repeatability, stability and recovery were good. Compared with the zero day, the content of cinnamic acid in the decoction of cinnamon decreased significantly (P<0.01) after 21 days and 30 days of ambient temperature storage and after 14 days, 21 days and 30 days of high temperature storage, but no significant change was found in the other groups (P>0.05). Conclusion This HPLC-DAD method had good stability and repeatability. Cinnamic acid was stable in the decoction of cinnamon for 30 days under refrigerated temperature.