Diameter at breast height is one of the most common and important stand parameters measured in forest inventory. In forest inventory, it is necessary to measure all trees within the sample area. In difficult terrain conditions, even this simple process of inventory becomes difficult. Therefore, it is important to follow the developing technology and integrate it into forest inventory. It is becoming possible to make various measurements from three-dimensional point cloud data provided by terrestrial lidar scanning and to provide some parameters of the tree or stand such as diameter at breast height from these data. In this study, the diameters of the trees within the sample area taken from the oak stand located in Düzce University Campus were first measured with a compass using the classical method and the values found were then compared with the values calculated from the lidar data. Although manual measurements can easily be made on a single tree basis in lidar data, using tools that automate the process in sample area-based studies increases efficiency. In this study, the determination of the diameter at breast height of trees from the point cloud was performed using 3DFin software. As a result of the analysis, it was confirmed that there were no significant differences between compass and lidar measurements at 95% confidence level.