LAG-3 cyclic peptide inhibitor C25 is a LAG-3 inhibitor. LAG-3 cyclic peptide inhibitor C25 has a high affinity with human LAG-3 protein with a K d value of 0.66 μM. LAG-3 cyclic peptide inhibitor C25 can block the interaction of LAG-3 with HLA-DR ( MHC-II ) and activate CD8 + T cells. LAG-3 cyclic peptide inhibitor C25 can be used in the study of cancer.