Abstract: The chemistry of forest precipitation was monitored in two different types of forests(Pinus tabulaeformis & Quercus variabilis mixed forest and Quercus variabilis pure forest)in Jiufeng National Forest Park, Western Beijing.Resultsshow that the pH value of precipitation ranges between 6.13 and 6.97 and no acid rain appeared; mean electrical conductivity(EC) was 0.18 mS/cm and the mean total suspended particles(TSP) is 44.02 mg/L, but these values changed significantly with stem-flow. Stem-flow shows acidification to different degrees(Pinus tabulaeformis> Quercus variabilis).The mean EC value of the stem-flow of Pinus tabulaeformis in the mixed forest is 1.00 mS/cm, 5.88 times larger than that of precipitation outside the forest. EC values of Quercus variabilis in pure and mixed forest show little difference, 0.34 mS/cm and 0.30 mS/cm respectively,2.00 and 1.76 times larger than that of outside precipitation. In summer, EC values of stem-flow of Pinus tabulaeformis decreases with the increase of times and with an increased amount of precipitation. The mean TSP of stem-flow in mixed forests is 116.95 mg/L, but 87.14 mg/L in a pure forest, 72.93 mg/L and 43.12 mg/L higher than that of precipitation.