The Dominican Republic has an important area of mountain forests, whose communities are highly threatened, and are also among the least studied tropical ecosystems with respect to the factors that determine their structure and composition of species. For this reason, the objective of this work was to evaluate the effect of the altitudinal gradient on the diversity and tree structure of the broadleaf forest of the Upper Yaque del Norte Basin. Sample units in three altitudinal ranges (RA) were analyzed, comparing their diversity and structure indices. 2800 individuals belonging to 34 families, 53 genera and 59 species were recorded. The diameter distribution of the individuals presented an arrangement of “inverted J” for the three altitudinal ranges. Differences in diversity were obtained for the altitudinal ranges, with RA1 presenting the lowest value in dominance and higher values in equity, richness and diversity, followed by RA3 and RA2 respectively, although these differences did not show statistical significance. The Upper Yaque del Norte Basin presented lower richness values than other tropical forests, mainly explained by anthropic actions. The forest structure variables did not show a relationship with the altitudinal gradient, obtaining a distribution whose trend would reflect the successional characteristic of the forest. Diversity indices were not related to altitude, which would be explained by the environmental heterogeneity of the study area.