Resumo. This study aimed at determining the age of demarcation of juvenile and mature wood of Luehea divaricata Mart., using anatomical characteristics. Three adult trees, in good trunk, with diameter at DBH larger than 30 cm, from Encosta Superior do Nordeste do Estado do Rio Grande do Sul, were selected. Discs with thickness of 2 cm at 0.1 m of height from the base of the trunk were used. From each disc a radial ribbon 2 cm wide including the pith was removed. The initial wood of each growth ring was separated for maceration (method of Jeffrey). There were measured length and diameter of thirty fibers from each growth ring. Demarcation of the two types of wood was defined by the radial variation (pith-bark) of the anatomical characteristics (length, diameter, width of the lumen and wall thickness of fibers), using simple linear regression. Results indicate that fiber length is the best characteristic, while fiber diameter, lumen width and wall thickness were considered inadequate for demarcation. The age of demarcation of juvenile and mature wood of Luehea divaricata was defined as 21 years-old, approximately.