The low proportion of gastric cancer (GC) patients with high HER2 expression limits the clinical application of trastuzumab, a humanized epidermal growth factor receptor 2 (HER2) antibody targeting for GC treatment. We found that Dicer was positively correlated with HER2 expression in GC tissue by immunostaining as well as induce HER2 overexpression without increasing invasiveness of GC cell. In addition, both the growth of GC referring to cell proliferation, invasion, migration and apoptosis was inhibited by Dicer overexpression. Moreover, the HER2 overexpression induced by Dicer provided more effective and additive target for trastuzumab to amplify the inhibition effect for GC cells in vitro and in vivo. Furthermore, as assessed in a subsequent experiment, calcitriol induced HER2 overexpression and amplified the inhibition effect of trastuzumab in GC cells referring to proliferation. Our finding demonstrated the calcitriol might increase indication of trastuzumab by inducing HER2 overexpression in GC patients. Dicer would be a potential target that extend the clinical indications of HER2 antibody in patients with low or negative HER2, who were not fit for HER2 antibody treatment before.