China’s 13th 5-year forestry plan explicitly requires the end of commercial harvesting and the improved management of natural forests by means of adjusting and optimizing forest structure and developing high-quality forest production. This requires the quantifi-cation of stand structure and species diversity. In this study, species distribution, association and size differentiation in a mixed broad-leaved/Korean pine (Pinus koraiensis Siebold& Zucc.) forest in Mudanfeng National Nature Reserve were analyzed using three spatial structure indices: uniform angle index, complete mingling index, and size differen-tiation index. The results show that the species were ran-domly distributed, with a moderate size differentiation and high mixed structure. Three dominant species exhibited different patts in terms of the three indices and their bivariate distribution. Overall, Korean pine showed an opposite trend to Manchurian maple (Acer mandshuricum Maxim.), whereas the distribution patt of Manchurian linden (Tilia mandshurica Rupr. & Maxim.) was similar to that of the entire stand. Our work contributes to further the knowledge of population structure in order to optimize the structure of secondary forests and plantations.