The bimetallic nanoparticles, used as catalysts due to their activity and selectivity, were prepared using one of the most versatile and convenient synthesis route: the alkaline polyol method. Mono and bimetallic nanoparticles, with both core-shell and alloy structure were obtained. The nanoparticles were characterized by TEM and XRD. The fractal dimension of each micrograph was computed using the "box-counting" method and the "information" method. Computing the box-counting fractal dimension at different grey-level thresholds one can obtain a specific curve. The paper will discuss the relation between the fractal dimension versus the grey-level threshold curve, the nanoparticles morphology and the preparation variables. The novelty of the work is that the shape of the fractal dimension versus the grey-level threshold curve is an indicator if the nanoparticles have alloy or core shell structure. Also, the paper will provide a method to compute both fractal dimensions: of the nanoparticles' cores and of the nanoparticles' shells. (C) 2018 Elsevier B.V. All rights reserved.