The vegetation is a mix of different forest types and habitats, with Northern tropical moist deciduous dominating some semi-evergreen patches. Sal is the dominant tree species here. The park also has extensive grasslands that are grazing grounds for many of the herbivores. These forests boast of many plants that have medicinal and aromatic properties.
The park is known for the tiger, elephant and hill mynah. It holds the highest tiger population in the state of Orissa. 55 species of mammals, 304 species of birds, 60 species of reptiles, 21 species of frogs, 38 species of fish and 164 species of butterflies have been recorded from the Park.
Apart from the tiger, the major mammals are leopard, sambar, barking deer, gaur, jungle cat, wild boar, four-horned antelope, giant squirrel and common langur. Grey hornbill, Indian pied hornbill and Malabar pied hornbill are also found here. The park also has a sizeable population of reptiles, which includes the longest venomous snake, the King cobra and the Tricarinate hill turtle. The Mugger Management Programme at Ramatirtha has helped the mugger crocodile to flourish on the banks of the Khairi and Deo Rivers.
Apart from its biodiversity, the region around Similipal forests is home to a variety of tribes. Prominent among these are Kolha, Santhala, Bhumija, Bhatudi, Gondas, Khadia, Mankadia and Sahara. Most of them are settled agriculturists, supplementing their income by collecting firewood and timber except for the last three who are indigenous hunter-gatherer communities living primarily off the forest, collecting forest produce. While the tribes earlier followed a number of traditional conservation practices like closed seasons, hunting taboos on specific species, maintenance of sacred groves (Jharia) etc., of late, these practices have been on the decline due to the increasing influence of modern civilization, increasing human population and decreasing wildlife availability.