The vascular flora of Middle Fork Woods Nature Preserve, Vermilion County, Illinois, was studied during the growing seasons of 1999–2002. A total of 376 species were found: 12 ferns, fern-allies, and gymnosperms; 99 monocots; and 265 dicots. The three families with the largest number of species were the Poaceae (44), Asteraceae (39), and Cyperaceae (34, 29 of which were members of the genus Carex). The overstory and woody understory of six forest communities were surveyed: a mature second growth dry-mesic upland forest dominated by Quercus alba (white oak) and Q. velutina (black oak); an old growth, dry-mesic savanna dominated by Q. alba with a dense understory of Acer saccharum (sugar maple); SW-facing and NE-facing forested slopes of a mesic ravine, both dominated by A. saccharum and Q. alba; a southern flatwood forest/ ephemeral pond community dominated by Q. bicolor (swamp white oak); and an immature second growth dry-mesic upland forest dominated by Q. alba and Carya ovata (shagbark hickory). Ground layer plants were surveyed in spring and fall in the dry-mesic upland forest communities, and for each species the relative cover, relative frequency, and importance values were determined. The preserve, comprised mostly of oak-hickory forest communities, was found to be in relatively good ecological condition. Mature and maturing second-growth oaks were present throughout much of the preserve. Sugar maples dominated the understory and, along with the dense overstory, were apparently limiting oak regeneration. The non-natives Lonicera spp. (bush honeysuckles), Elaeagnus umbellata (autumn olive), Alliaria petiolata (garlic mustard), and the native sugar maple, will continue to need to be controlled. Prescribed fire is recommended to limit populations of these species.