The increase in exploration has been changing the structure and diversity in the Caatinga fragments. The objective of this study was to evaluate the floristic composition, structure and similarity between caatinga fragments in the state of Piauí. Forty-two square plots were located 400 m2 (20 × 20m), divided into two areas, 20 plots in area I and 22 plots in area II. All living individuals with circumference at breast height (≥ 5 cm) were measured. In area I, 2,080 individuals were sampled distributed in 15 families, 34 genera and 43 species. The Shannon diversity index (H') was 2.77 nats.ind-1 and equability (J) 0.73. In area II, 1,275 individuals were measured in eight families, 20 genera and 26 species, the Shannon index (H') was 1,70 nats.ind-1, pielou's equability value 0,52. The Fabaceae family was the most representative in number of species. The two areas share few species six (9,5%), area I has the highest percentage of exclusive species 37 (58,7%) in relation to area II. The two areas have differences in floristic and structural parameters. Species diversity was considered low for normal pattern in Caatinga areas. Area I was higher than area II in density, number of families, species and diversity index, as demonstrated by the high diversity β found, are in different state of conservation and ecological succession.