Abstract: In order to understand the structure of tree diversity in the city centre of Dublin, the tree diversity distributed in the St. Stephen′s Green, Merrion Square, Iveagh Gardens and Fitzwilliam Square were investigated, which were also analyzed by the parameters of relative importance, diversity indices and similarity coefficient. Results showed that there were 65 species (or variants), belonging to 24 families 43 genera, which of them were in the main from Rosaceae, Fagaceae, Aceraceae and Betulaceae. Among these species, the proportion of deciduous and evergreen trees in the St. Stephen′s Green, Merrion Square, Iveagh Gardens and Fitzwilliam Square is about 1 : 5, 1 : 5, 1 : 4 and 1 : 5, respectively. From the viewpoint of similarity coefficient, there was a low similar on the species composition among the parks, but there was closer similarity coefficient between the larger parks, the same as the smaller ones. The integral population structures were widely distributed. The values of relative importance indicated that Platanus hispanica, Ilex aquifolium, Quercus ilex, Acer pseudoplatanus, Tilia sp. and Aesculus hippocastanum; meanwhile, number of 2 or 3 trees species constituted the main body in each parks. Based on the analysis of the dominant tree species, P.hispanica and A.pseudoplatanus were planted in the early stage, which of them were the most important species in the parks. According to the 4 indices of species diversity, the result of Merrion Square was better than the 3 others.