Resumo. This study was undertaken to evaluate the fiber dimensions and the transition age between juvenile and mature wood of Eucalyptus grandis . It was selected, based on the Assmann’s dominant diameter, four trees of an experimental stand located in the northern coast of Rio Grande do Sul state, with 18 years of age. After cutting down, in the disk corresponding to the region of the DBH (diameter at breast height) were marked and sectioned central baguettes, where crafted fragments were selected at regular intervals in pith-to-bark direction for maceration and, thereafter, determination of the length, the total diameter, the lume diameter and the wall thickness of the fibers with the aid of an optical microscope with a graduated eyepiece. The segregation of both types of wood was determined by the radial variation of the fiber length and using simple and multiple linear regressions; and a transitional age through simple linear regression. The lower values for the dimensions of length, total diameter and wall thickness of the fibers were observed near the pith, with an increasing trend toward the bark, suggesting the existence of differences between wood near the pith and the bark. However, this standard was not observed in the lume diameter of the fibers, which showed an increase to the middle position of the radius followed by a decrease to the outer portion. The transition zone between juvenile and mature wood has been established between the ages of 4.2 and 4.7 up to 8.1 and 9.1 years, range termed juvenile wood that corresponding to 4.2 and 4.7 years; and adult wood exceeding 8.1 and 9.1 years. The transition between adult and juvenile wood occurred at 10 cm from the pith, that is, in the age of about 6.1 years. The definition of the age at which the transition between the juvenile and mature wood occurs facilitates the wood transformation processes once the juvenile portion constitutes an obstacle to the viability of its industrial use.