Trastuzumab, a humanized monoclonal antibody derived from recombinant DNA, is used in patients with breast cancer with HER2 gene amplification. The survival benefit from trastuzumab has been well established in patients with early and metastatic breast cancer who had over expression of HER2. We reported a case of severe thrombocytopenia after eight cycles of trastuzumab treatment for breast cancer. Before the 9th trastuzumab treatment, the patient's platelet decreased to 48 × 109/L. Recombinant human thrombopoietin was used, and the platelet level increased to normal level. Before the 10th treatment, the platelet count of the patient was 99 × 109/L. However, during the 10th and 11th trastuzumab treatment, the platelet count decreased to 5 × 109/L in 24 h. After treatment with TPO and corticosteroids, the platelet levels increased to the normal level in 7 days. Trastuzumab-induced thrombocytopenia is rare but still occurred even after 8 cycles of trastuzumab treatment.