Resumo. ABSTRACT Urban forest fragments surrounded by housing matrix are a source of richness and can function as a conservation site in the relation between the living beings and the environment. The present study have as aim to describe the community structure of tree and arborescent species in a urban forest fragment in the municipality of Sério, Rio Grande do Sul (RS) state, inserted in the Atlantic Forest biome, phytoecological formation Montane Deciduous Forest. The floristic inventory was produced through walking method, reaching the total fragment area, in monthly field incursions. For sampling, 40 sampling units (UA’s) of 100 m² were scattered, with distance of 20 meters in between them. Inside the UAs, it was obtained the height and circumference at the chest height (CAP) of all specimens which have CAP equal or higher than 15 cm. It was estimated the parameters of dominancy, density, frequency, absolute and relative, and the importance value index (IVI). The floristic inventory found 59 trees and arborescent species belonging to 48 genera and 30 families. In the quantitative inventory, 535 tree specimens belonging to 42 genera and 27 families were sampled. Among the species, three are exotic and three are in the threatened species list of the state of Rio Grande do Sul. The absolute density was estimated in 1,338 individuals/ha -1 . Actinostemon concolor showed the denser values (140 ind.ha -1 ), followed by Alchornea triplinervia e Eugenia rostrifolia , both with 108 ind.ha -1 . The species with the great IVI’s were Alchornea triplinervia , Vernonanthura discolor , Actinostemon concolor , Eugenia rostrifolia and Araucaria angustifolia. The estimated diversity found through the Shanon Index (Hʼ) was 3.35, and the equability of Pielou (Jʼ) was 0.85, denoting a consistent species diversity. The fragment can be seen as an important shelter for the regional wild fauna and flora. However, to still working in its ecological roles we concluded that conservation and protection measures are needed in this area, being the removal of exotic species the first step to be taken..