Serengeti is the largest ecosystem in Tanzania endowed with high level of biodiversity in protected and unprotected areas. Serengeti National Park is the only protected area in a matrix of unprotected areas characterized with a wide range of human activities which threaten biodiversity conservation. It was assessed plant species composition, diversity, distribution and vegetation structure in both the protected and unprotected areas of the ecosystem. Two transects with twenty plots each were established in each vegetation type in which data was collected. A significantly higher species composition (262) and diversity (2.39 ± 0.03) was observed in unprotected areas than in the protected area (163 and 2.06 ± 0.04 respectively). The DCA ordination of the species data from the two management regimes formed four clusters based on similarity in plant species composition, where three clusters were from unprotected areas and one from protected area. The variations in plant species composition ranged from 35.69% to 65.92% for the first four DCA axes. There was no significant difference in stem density between protected and unprotected areas of the ecosystem. Although high density of trees at 10 - 14 cm DBH sizes was observed in both sides of the conservation management regimes, individuals with DBH above 65 cm existed in the protected area but were absent in unprotected areas. It can be concluded that the decrease of plant species diversity and density in unprotected area is because of habitat degradation associated with exploitation pressure, livestock grazing and other forms of disturbance related with anthropogenic activities. Because of high demand for plant resources, the local community needs to establish woodlots to cater for wood resource needs outside the protected areas of Serengeti. Also, pastoralists need to diversify livestock keeping systems compatible with limited grazing land in unprotected area of the Serengeti ecosystem.