Urban forests form an important component of the urban environment. They make a significant contribution to landscape silhouettes due to species diversity and the differences observed within forest communities such as tree composition, size and form. Visual effects such as these are referred as aesthetic value. Therein, quantitative assessments of targeted forest structures that will optimize aesthetic values are vital during urban forest planning. However, the silhouette effect, which is an important component of aesthetic value, has not been extensively studied to date. This study aims to reveal the relationship between forest structure, the visual quality of tree composition and the different forms and sizes that create the silhouette effect. To this end, virtual landscapes of different structures were created using computer-assisted images. The forest structure was characterized by a Form Mingling Index (FMI) and Height Dominance Index (HDI). A Pearson correlation was applied in order to find the relationship between the landscape silhouette values through a perceptual test as well as the FMI and HDI. The results of the analyses showed that there was a strong positive relationship between the FMI and HDI (r = 0.78, r = 0.75, P < 0.01, respectively) and landscape silhouette values. The combined effect of FMI + HDI was found to have the highest relationship with landscape silhouette values (r = 0.80, P < 0.01). In conclusion, FMI and HDI offer a rapid, cost-effective method which can be used by managers to assess the silhouette value of an urban forest landscape.