The paper presents the results of the studies on the species richness and the dissimilarity of the flora of vascular plants from the mineral-habitat islands which are surrounded by the mires in the Upper Narew Valley (NE Poland). The moderately strong relationship found between the size of the islands and the number of plant species is probably an effect of the land-use diversity. Floristic value is dependent on the island size, but the analogous relationship was not revealed for floristic dissimilarity. The fairly distinct reaction of the floristic richness on the area and the perimeter of the islands was confirmed in the case of the species of mineral habitats and these of the mire ones.