A technique of analysis of images coupled with a data processing system using VISILOG 5.2 software was used to determine the morphological characteristics of flocs obtained in natural organic matter (NOM) extracted from the waters of Keddara dam (supplying partially Algiers city, Algeria) and from synthetic fulvic acids. Slower sedimentation speeds (approximate to 0.7 mm.s(-1)) as well as the small effective volumic masses (approximate to 3 kg.m(-3)) of the floes of NOM extracts from Keddara dam show poor sedimentation capacity. The fractal approach, involving natural extracts, showed that fractal dimensions obtained from two proposed relations are close (D-f2 = 1.55-1.51 and D-beta = 1.99-1.91 1). Results reveal that, from an applied point of view, in spite of the optimization of the conditions of coagulation-flocculation, the NOM of Keddara dam can partially remove the advantage of optimisation.