Abstract: AIM: The synthesis and antiaggregation activity of a series of compounds with p-amidinophenyl group and tyrosine methyl esters as their terminals were studied. METHODS and RESULTS: A series of O-(4-aminoiminomethylphenyloxy)alkyl-N-substituted-L-tyrosine methyl esters were prepared from 4-cyanophenol, L-tyrosine and different dihalo-alkanes. All compounds (IVa～i) were new compounds. Their in vitro activity against ADP-induced platelet aggregation were tested. The structure-activity relationships were discussed. CONCLUSION: Some of the compounds showed antiaggregation activity on platelet rich-plasma at the final concerntration of 1×10^-6 mol.L^-1. The potency of IVa～i showed that compounds with three methylene groups are better than those with four methylene groups on the same R-substituent in the above series of compounds.