Abstract: Through stem analysis of sample trees, the biomass, growth process, and diameter structure of two 21 years old Pinus tabulaeformis forests growing on the shady and sunny slopes in Loess Plateau hilly and gully regions were investigated. The results showed that there were distinct differences between these two forests in their tree height, diameter at breast height (DBH), and timber volume. The forest biomass, growth status, and diameter structure on shady slope were superior to those on sunny slope. The fast--growing period of tree height all appeared in the period of 9-13 years, and after 13 years, the annual increment of tree height was larger in shady slope forest than in sunny slope forest, with the increment in the 21^st year being 0.26 and 0.1, respectively. The DBH growth of the two forests was declined greatly after 13 years, but the decrement was obviously less in shady slope forest than in sunny slope forest. After 17 years, the annual increment of DBH growth was larger in shady slope forest than in sunny slope forest, with the values being 0.46 cm·a^-1 and 0.27 cm·a^-1, respectively in the 21^st year. Before 13 years, there was little difference in the increment of timber production between the two forests, but the increment after 13 years was larger in shady slope forest than in sunny slope forest,with the values of 0.0023 m³ and 0.0015 m³, respectively in the 21^st year . The summit of DBH distribution curve was all partial to left, but the skewness of shady slope forest (SK=0.75) was lower than that of sunny slope forest (SK=1.03), and the kurtosis of shady slope forest (K=1.05) was higher than that of sunny slope forest (K=0.94), indicating that sunny slope forest had a larger stand density than shady slope forest.