Resumo. The degradation status of the forests in western Santa Catarina state reflects in a reduced number of conserved areas that enable studies on forest communities diversity and structure in order to create strategies for ecosystems preservation, management and restoration. The objective of this study was to describe the diversity and structure of a forest remnant of Seasonal Submontane Deciduous Forest located in the municipality of São João do Oeste, Santa Catarina state. For the inventory, we demarcated 102 sampling units of 10 × 10 m, continuously arranged in a grid of 6 × 17 plots, in which all plants of the arborescent-arboreal sinusiae with perimeter at breast height (PBH) ≥ 15 cm were sampled. We surveyed 1441.18 living trees and 93.14 standing dead plants per ha. The 79 species found belong to 61 genera and 33 families. Sorocea bonplandii , Nectandra megapotamica , Inga marginata , Ocotea diospyrifolia and Trichilia claussenii presented the highest importance value indexes. The Shannon diversity index (H’) was 3.21 and the Pielou equability (J’) was 0.735. The vertical structure of the vegetation sampled was divided into treelets (2-11 m), canopy (12-17 m) and emerging trees (18-25 m). The studied forest fragment is an important reference ecosystem for the implementation of strategies for preservation, management and restoration of seasonal forests.