Resumo. The Mixed Ombrophylous Forest is one of the most threatened forest ecosystems in the country and it is estimated that only about 3% of the original covering of this vegetation type has remained. In Minas Gerais state, these remaining forests are rare, restricted mainly to the south of the state. This study aimed to evaluate the structure, diversity and heterogeneity of tree layer species of a Mixed Ombrophylous Forest in its most extreme northern distribution (Minas Gerais state, Brazil). The study was conducted in Parque Estadual da Serra do Papagaio (PESP), which includes the only remaining of araucaria forest in a full protection Conservation Unit (CU) in Minas Gerais state. We allocated in the area 25 permanent plots of 20m x 10m (0.5 ha) and every shrub and tree vegetation with DBH ≥ 4.8 cm was measured, identified and the height was estimated. We measured 1,158 individuals, belonging to 41 species, 28 genera and 22 families. It was observed a community with a strong concentration of the importance values (IV) in the first three species (Podocarpus lambertii, Araucaria angustifolia and Myrceugenia bracteosa), that together add up 52.74% of total IV of the community. The basal area of the community was 30.82 m² (61.65 m².ha-1), being one of the largest ever registered for Mixed Ombrophylous Forests. The Shannon species diversity index (H’ = 2.67 nats.ind-1) was in the range observed in other studies of Upper Montane Mixed Ombrophylous Forest. Multivariate analysis indicated a low floristic heterogeneity in the community; however, this low heterogeneity occurred because the main species in terms of density are also the most frequent and are distributed throughout the forest. The study shows a well-structured community with large accumulation of biomass, showing the importance of the conservation unit for the preservation of this important, rare and threatened vegetation type in Minas Gerais state.