Background: Trastuzumab (Tmab) is an active molecular-targeted drug for HER2-positive gastric cancer (GC) patients. HER2 expression is known to change during the treatment of Tmab and HER2 amplification in blood is widely investigated as a new biomarker of the treatment candidate or the monitoring of HER2-target therapy. We evaluated the change of the HER2 expression by immunohistochemistory (IHC) and HER2 copy number by degital PCR during the treatment with Tmab in metastatic HER2-positive GC patients. Methods: Metastatic HER2-positive GC patients treated with Tmab were registered prospectively, and tissues were obtained by biopsy from primary lesions at the following points: (1) pre-treatment, (2) post-treatment, and (3) disease progression during chemotherapy with Tmab. Formalin-fixed paraffin-embedded tissue slides were prepared, and the expression of HER2 expression were scored by IHC and HER2 copy number were evaluated by digital-PCR. Results: Among 20 enrolled patients, HER2 expression was evaluated by IHC in all patients. HER2 copy number was evaluated by digital PCR in 15 patients. A patient was excluded because HER2 expression were not detected by re-evaluation. The median of HER2 copy number of 33 sampling points during the treatment was 6.37 (range: 2.12 - 85.8). The median of HER2 copy number of IHC 3+, IHC 2+, IHC 0-1 were 22.4 (3.37 – 85.8), 2.88 (2.65 – 3.6), 2.37 (2.12 – 3.85), respectively. High level of HER2 copy number was detected in tumor tissues of HER2 IHC 3+ compared with that of IHC≦2+. HER2 expression by IHC was disappeared after treatment of Tmab in 42% of HER2-positive patients. Among these patients, HER2 copy number at pre-treatment were relatively low (2.63 - 3.6). Conclusions: High copy number of HER2 was detected in tumor tissues with HER2 IHC3+ in HER2-positive GC patients during the treatment of Tmab. In addition, low copy number of HER2 in tumor tissues at pre-treatment may be associated with the loss of HER2 expression after treatment of Tmab. Further research by large number of patients to confirm our results is required.