Abstract: The objective of this work was to develop a bioassay to quantify the antiplatelet aggregation activity of hirudo for quality evaluation and control. Antithrombin activity of hirudo extracted by high temperature decoction was determined by thrombin titration. Antiplatelet aggregation activity of hirudo was determined through pharmacodynamic experiments in vitro and in vivo using a bioassay we developed for quantifying inhibition of platelet aggregation. Methodological investigation was carried out and the titers of 12 batches of hirudo samples were determined. During the experiment, the disposal of animals is in accordance with the ethical standards of animal experiments. The results showed that the antithrombin activity of hirudo decocted at high temperature decreased significantly and almost lost its activity. Hirudo inhibited platelet aggregation and results in vivo and in vitro were consistent. These assays were employed to test 12 batches of hirudo. The results demonstrated that the biopotency of 12 batches was 113.49, 96.13, 121.22, 127.33, 83.48, 108.72, 131.41, 127.95, 76.90, 126.27, 132.89 and 573.53 U·mg^-1. The method was reliable and reproducible and can be used to assess the quality of hirudo.