Trastuzumab is a recombinant antibody drug that is widely used for the treatment of HER2‐overexpressing breast carcinoma. Despite encouraging clinical results, many HER2‐overexpressing carcinomas are primarily resistant to trastuzumab. We attempted to explain trastuzumab resistance and search for solutions. Since the killer cell lectin‐like receptor G1 (KLRG1), an inhibitory receptor expressed on subsets of natural killer (NK) cells recognizes E‐cadherin as ligands and may inhibit immune responses by regulating the effector function of NK cells, we used HER2‐overexpressing carcinoma cells which were expressing E‐cadherin to investigate the role of antibody‐dependent cellular cytotoxicity (ADCC) through KLRG1 on NK cells in vitro and vivo. The results indicated that HER2‐overexpressing carcinoma cells were killed by trastuzumab‐mediated ADCC and the ADCC activity was reflected the degree of E‐cadherin expression on carcinoma cells. We found that expression of E‐cadherin was shown to be a predictor of response to trastuzumab‐based treatment for HER2‐overexpressing carcinomas, furthermore, trastuzumab‐mediated ADCC was markedly enhanced by KLRG1‐negative peripheral blood mononuclear cells (PBMCs^KLRG1(−)).