National power grid in Poland requires modernisation, therefore by 2030 numerous actions are scheduled regarding the expansion of the existing network and the construction of new electricity transmission lines (power lines). The planned activities will undoubtedly change the quality of landscape and result in fragmentation of forest habitats, some of which are characterised by high biodiversity and constitute a key element of ecological corridors. The aim of the present paper was to outline the issue of the impact of the power line corridors on forest habitat fragmentation, as well as to present the GuidosToolbox software (Graphical User Interface for the Description of image Objects and their Shapes) used, among other things, for the determination of the degree of fragmentation of forest habitats. The analysis concerned the Puszcza Bukowa forest, which is a part of the Natura 2000 network and is protected under the Szczecin Landscape Park ‘Puszcza Bukowa’. Despite abundant natural assets of the Puszcza Bukowa forest, it was necessary to run many power lines through its area due to location of the forest in the vicinity of the Szczecin agglomeration. The course of power lines contributed to the fragmentation of the discussed forest complex and to the depletion of its interior classified with the GuidosToolbox software as ‘Intact’. The software discussed in the present paper may prove useful in the identification of the degree of forest area fragmentation, connected with the course of high-voltage power transmission lines, and in the assessment of the impact of the planned investment projects on biocenoses.