Fucoidans and fucans are a family of sulfated homo and heteropolysaccharides respectively, composed mainly of a-(1®2) and a-(1®3) linked by L-fucose residues. Fucans contain other sugar as galactose, mannose, glucose and xylose. Properties such as the ability to act as an anti-contraceptive, anti-inflammatory, and anticoagulant and anti-tumor agent are related to these polysaccharides.  We have focused our attention on the platelet aggregation of purified fucoidans (F1, F2 and F3) and no purified FT from Fucus vesiculosus. These compounds were obtained from fractionation of the fucoidan with different concentrations of acetone (1, 2 and 3 v). The MW of the F1, F2 and F3 were 170, 117 and 15 KDa and the sulfate content was (54, 43 and 26%) respectively. Tests of agregation platelet by turbidimetric method were performed (at 37 º C by 7 minutes) in the platelet aggregometer PACKS-4® using the using citrated human blood. Our results demonstrate that the aggregation platelet with fucans fractions FT, F1, F2 and F3 exhibited a two-phase answer in the concentration of 250 µg/mL with maximum aggregation of 76.36 ± 10.3 % ; 69.54  ± 9.40 %; 75.94  ± 9.01 %; 51.13 ± 9.59 % respectively. When collagen was used (5 ug/mL) it was also observed a high aggregation (95.9 ± 6.71 %). However, the tests with ADP (3 µM) showed a monophase curve with maximum aggregation (81.53±10.73 %). On the other hand, at the concentration of 5 µg/mL was observed a hipoagregate profile for the fractions FT (15.17±5.2) and for the fraction F2 (19.1±5.41%).  Platelet aggregation was not verified for the fractions F1 and F3. The results obtained demonstrate that the fucans induced the aggregation of platelet in high concentrations. However, in low concentrations they have contrary effect. Future studies will have to be made with objective to elucidate the molecular bases of this process.