The detailed analysis and modelling of microclimate in the urban space has gained importance with the emergence of the urban heat island effect in recent years. ENVI-met is one of the commonly used microclimatic models to the microclimate variables of an urban fabric. Urban green spaces positively affect the urban climate. However, characteristics of these spaces within an urban fabric (the area size, location in the city, tree cover density, etc.) are important to change urban microclimate. Therefore, the aim of this study is to investigate the statistical relationship between the area size and the effect of the tree cover density in Adana City, Turkey. Three scenarios were primarily determined in order to explain the effect of the tree cover density on the urban heat island. Secondly, PET values were calculated for each urban green space by ENVI-met. Lastly, the mean PET values of each area size class for different tree cover were statistically analysed by ANOVA-Tukey HSD. Consequently, this paper aims to determine the suitable tree cover density for urban green areas and develop suggestions for planning and design strategies.