Natural killer cell-based immunotherapy has become a leading-edge tool against cancer, but still faces a variety of challenges, such as phenotype shift and dysfunction of NK cells in tumor microenvironment. Thus, finding potent agents that could inhibit the phenotype shift and incapacity of NK cells in the tumor microenvironment is essential for improving antitumor effects. dl-tetrahydropalmatine (dl-THP), one of the active alkaloids of Chinese herb Corydalis Rhizoma, has been proven to possess antitumor activity. However, whether dl-THP acts on NK cells to enhance antitumor activity remains unknown. In this study, we found that the proportion of blood CD56(dim)CD16(+) NK cells was decreased while the proportion of CD56(bright)CD16(-) NK cells was increased when the cells were cultured in conditional medium (CM, medium from the human choriocarcinoma cell lines JEG-3). dl-THP could alter the varied proportion of CD56(dim)CD16(+) NK cells and CD56(bright)CD16(-) NK cells in CM respectively. Importantly, the expression level of NKp44 on CD56(dim)CD16(+) NK cells was dramatically reduced when the cells were cultured in CM, which could also be reversed by dl-THP. Furthermore, dl-THP increased the decreased NK-cell cytotoxicity when cells were cultured in CM. In summary, our study demonstrated that dl-THP could recover the decreased NKp44 expression level on CD56(dim)CD16(+) NK cells and restore the cytotoxicity of NK cells in tumor microenvironment. CI - Copyright (c) 2023 Elsevier GmbH. All rights reserved.