To develop a GC-MS method for the determination of cinnamaldehyde and study on the influence of decoction methods and compatibility on the contents of cinnamaldehyde in Mahuang decoction.Gas chromatography-mass spectrometry (GC-MS) was used. Chromatography/mass spectrometric conditions were: capillary column DB-17 (30 m x 0.5 mm, 0.25 micron), helium as carrier gas, non-split injection, column primary temperature 120 degrees C, 120 degrees C 8 degrees C/min-->250 degrees C, with 1.0 ml/min runoff, and injection volume 1 microliter. The results were analyzed by one-way ANOVA (SPSS).The calibration curves were linear in the range from 0.0136 mg/ml to 0.136 mg/ml (r = 0.9999). The RSD of precison, stability and reproducibilities were 1.02% (n = 6), 2.57% in 24 h and 4.20%, respectively. The cinnamaldehyde content in the blended Mahuangtang decoction (BMHD) was significantly lower than the one in the separated and mixtured decoction and Ramulus cinnamomi decoction (P 0.05).The main influencing factor of the cinnamaldehyde content was the compatibility. This method is precise and reliable to study cinnamaldehyde in Mahuang Decoction.