The human epidermal growth factor receptor-2(HER2) is overexpressed/amplified in a number of cancers. HER2 is implicated in disease initiation and progression and associated with poor prognosis. Trastuzumab(Herceptin) is a recombinant DNA-derived humanized anti-HER2 monoclonal antibody that selectively binds with high affinity to extra-cellular domain. In vitro assay, trastuzumab has been shown to inhibit the proliferation of human tumor cells that overexpressed HER2 and to be a mediator of antibody-dependent cellular toxicity. Clinical trials have demonstrated that it is effective as both single and combination with chemotherapeutic agent in recurrent and metastatic breast cancer patients who's tumor tissue are overexpressed HER2. The incidence of severe adverse events were low but the occurrence of cardiac toxicity was unexpectedly high if trastuzumab was combined with anthracycline containing chemotherapy. The further studies are underway to assess the role of trastuzumab in combination chemotherapy, adjuvant chemotherapy and other type of tumors.