A purified fucoidan SF (348 kDa) was isolated from Sargassum horneri. Six low molecular weight fragments SFs (75-8.1 kDa) were prepared by acid degradation from SF. Anticoagulant properties of SF and SFs were evaluated. SF and SFs with 75-17 kDa had better anticoagulant activities with the activated partial thromboplastin time (APTT) greater than 120 s at 200 μg/mL. The decrease in the molecular weight caused a decrease in anticoagulant activity, and a slight decrease almost resulted in loss of anticoagulant activity when below 17 kDa (APTT < 59 s). Angiotensin-converting-enzyme (ACE)-inhibitory activities were also detected. Only when the molecular weight dropped to a certain extent (8.1 kDa), the fucoidan showed ACE-inhibitory activity (IC₅₀=2.16 mg/mL). Structure analysis indicated the main change of SFs was a reduction in molecular weight. Thus molecular weight is an essential factor affecting the bioactivities. Longer chains were essential to complete thrombin inhibition, whereas ACE-inhibitory activity required shorter ones.