Objectives: To compare the contents of hazardous substances before/after a decoction. Methods: The heavy metal contents before/after a decoction were measured by Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES) and mercury analyzer. In order to analyze pesticides in 6 samples we used simultaneous multi-residue analysis of pesticides by GC/ECD, which was followed by GC/MSD analysis to confirm the identity of the detected pesticide in each sample. In addition, the contents of sulfur dioxide (SO2) were performed by Monier-Williams distillation method. Results: 1. The mean values of heavy metal contents (mg/kg) for the samples were as follows: Socheongryong-tang (before decoction-Pb; 1.115, Cd; 0.179, As; 0.069 and Hg; 0.028, after decoction-Pb; 0.110, Cd; 0.011, As; 0.005 and Hg; 0.002), Insampaedok-san (before decoction-Pb; 1.207, Cd; 0.148, As; 0.171 and Hg; 0.026, after decoction-Pb; 0.075, Cd; 0.006, As; not detected and Hg; 0.001), Oryung-san (before decoction-Pb; 1.955, Cd; 0.430, As; 0.063 and Hg; 0.027, after decoction-Pb; 0.083, Cd; 0.013, As; 0.006 and Hg; 0.002), Hwangryunhaedok-tang (before decoction-Pb; 1.825, Cd; 0.210, As; 0.050 and Hg; 0.009, after decoction-Pb; 0.107, Cd; 0.010, As; 0.005 and Hg; 0.001), Bangpungtongseong-san (before decoction-Pb; 1.740, Cd; 0.162, As; 0.585 and Hg; 0.018, after decoction-Pb; 0.041, Cd; 0.006, As; 0.022 and Hg; not detected) and Oyaksungi-san (before decoction-Pb; 1.199, Cd; 0.183, As; 0.321 and Hg; 0.031, after decoction-Pb; 0.096, Cd; 0.008, As; 0.021 and Hg; 0.0004). 2. Contents (mg/kg) of sulfur dioxide (SO2) before a decoction in Socheongryong-tang, Insampaedok-san, Oryung-san, Hwangryunhaedok-tang, Bangpungtongseong-san and Oyaksungi-san exhibited 3.2, 5.7, 4.5, 49.8, 7.8 and 22.4, respectively. However, contents of sulfur dioxide after a decoction in all samples were not detected. 3. Contents (mg/kg) of residual pesticides before/after a decoction in all samples were not detected. Conclusions: These results will be used to establish a criterion of heavy metals, residual pesticides and sulfur dioxide.