Plant composition of urban green spaces is an important component of urban ecosystem as it influences the provision of many environmental and social services that contribute to the quality life. In Nairobi, a few remnants of continuous highland forest exist but they are under increasing pressure from the rapidly changing surrounding landscape. The plant composition is being altered by human encroachment and other related activities. The status of the current plant composition in relation to location and disturbance level is unknown. This study was therefore carried out to determine the variation in tree composition and distribution in three major green spaces within Nairobi city namely City Park, Karura and Ngong’ forests. Transects were laid out along environmental gradients, and the type, size, abundance and diameter at breast height (DBH) of tree species recorded within 20*15 m quadrats. The following aspects were calculated; abundance, species richness and distribution of tree diameters at breast height (DBH) and importance value (IVI). Indigenous species contributed 82% whilst exotic species accounted for 18% of the total species recorded. A mean quadrat species richness of 6.3, 4.7 and 4.1 was recorded in City Park, Karura and Ngong’ forests, respectively. It was observed that few tree species dominate and this reduces the diversity. At forest edges, exotic species were abundant, but this changed as one moves to the center, where the composition was mainly indigenous due to minimal disturbance. It can be concluded from this study that for conservation of the green urban spaces, there should be proper planning in place to minimize the human encroachment and to enhance plant diversity especially indigenous species. Further, it is necessary to encourage all stakeholders to participate in the conservation of these important sites.