Urban green spaces’ structure and function can promote human health and environmental quality and also decrease some challenges that have been increased due to urbanization. So, it is important to do some studies to investigate the urban trees’ diversity and structure because it can not only raise environmental awareness but also help policymakers for better management to achieve the desired environment. According to our knowledge, there has been done no investigation on the diversity of the urban trees of Tehran. The present study aims to assess the current structure and diversity of trees in Tehran green spaces (as a case study), using the i-Tree Eco model. Data from 316 field plots were collected during the growing season. It must be noted that the primary number of plots was 330, but some of them were not accessible. The plots were selected based on a randomized cluster sampling method and analyzed using the i-Tree Eco model. The results indicate that the overall tree density in Tehran is about 97 trees/ ha and the number of trees is about 7,127,933. The most common tree species are Afghan pine (13.0 percent), Black locust (10.0 percent) and, Arizona cypress (9.4 percent). In Tehran, about 28 percent of the trees are Asian endemic species and most trees have American origin (26 percent of the trees). Tehran urban trees are composed of endemic and exotic tree species. Therefore, Tehran's urban forest often has a diversity that is higher than the surrounding native landscape. Thus, the paper illustrates a methodology to assess the current structure and trees’ diversity of urban forests, which helps the development of future urban tree planting strategies and quantitative assessment.