Abstract: In recent years, the influencing mechanism of topography and competition on tree growth is getting more attentions. The research was conducted in a 9 ha dynamic monitoring plot of typical mixed broadleaved-Korean pine (Pinus koraiensis) forest in Liangshui Natural Reserve, Heilongjiang Province of northeastern China. Based on field investigation data in 2010 and 2015, this study analyzes the influences of topography (i.e., slope aspect, slope gradient, slope position) and competition among trees on the increment of diameter at breast height (DBH) for major tree species in mixed broadleaved-Korean pine forest. Results showed that the annual average DBH increment of diameter classⅠ(1 cm≤DBH < 10 cm) for Tilia amurensis, Acer mono and Abies nephrolepis as well as diameter class Ⅱ(DBH≥10 cm) for Pinus koraiensis, Tilia amurensis, Acer mono, Fraxinus mandshurica were all significantly correlated with topography (P < 0.05). The competition had significant influences on the annual average DBH increment of Pinus koraiensis, Tilia amurensis, Acer mono, Fraxinus mandshurica, Abies nephrolepis, Betula costata, Acer tegmentosum for diameter class Ⅰ as well as diameter class Ⅱ for Pinus koraiensis, Tilia amurensis, Acer mono, Abies nephrolepis, Ulmus japonica (P < 0.05). Generally, the annual average DBH increment of shade-tolerant and non shade-tolerant groups was significantly influenced by both topography and competition in mixed broadleaved-Korean pine forest. The DBH increment of large diameter classes of trees was mainly influenced by the topography, while the DBH increment of small diameter classes of trees was mainly influenced by the competition.