Abstract: A bioassay method for inhibiting platelet aggregation in vitro was established to quantify the pharmacological effects of Compound Danshen Tablets and support its quality control. The inhibition of platelet aggregation in rabbit plasma in vitro by Compound Danshen Tablets was used as the experimental system. The titer was calculated by using the method of dose-response parallel lines. As a result, a bioassay for the inhibition of platelet aggregation in vitro by Compound Danshen Tablets was established. Linearity was good in the concentration range of 0.128 g·mL^-1 to 0.205 g·mL^-1, and the titer of standard Compound Danshen tablets was 7 659 U·g^-1 according to the titer definition. This in vitro assay was simple, reliable, reproducible and convenient. The activity of Compound Danshen Tablets in inhibiting platelet aggregation was quantified by potency assay and the quality of different batches was evaluated. The method can be applied for the quality control of Compound Danshen Tablets.