Bioassay-directed fractionation from the methanol extract of the fresh herb of Cassytha filiformis led to the isolation and characterization of six aporphinoid alkaloids, actinodaphnine (1), N-methylactinodaphnine (2), cathafiline (3), cathaformine (4), predicentrine (5) and ocoteine (6). They showed potent inhibitory activity of rabbit platelet aggregation induced by adenosine diphosphate (ADP), arachidonic acid (AA), collagen and platelet- activating factor (PAF) factor. Alkaloids 1 and 2 also exhibited strong inhibition of aortic contraction induced by K+ (80 mM) and norepinephrine (3 μM). Moreover, 3 and 4 are two novel aporphinoid alkaloids possessing an N- (methoxycarbonyl) group.