Seed size is one of the factors that influence germination and the vigor of the seedlings. Thus, were studied the germination and the vigor of seedlings of Acacia senegal (L.) Willd. in function of different seeds size classes, which were classified with the use of circular sieves (meshes 14, 15, 16, 17, 18, 19 and 20). Were appraised the germination percentage and the fresh matter weights and medium lengths from seedlings, primary roots and hypocotyls. Determinations were also made for the seeds medium length (mm), width (mm) and thickness (mm). The experimental design was completely randomized blocks, being constituted of seven treatments, in four repetitions of 25 seeds each. The percentage and germination speed index were not affected by the size of the seeds. The primary root and seedlings medium length increased with seed size as well as the weight of the hypocotyl and primary root. Seeds size influenced more the vigor than the germination.