PD-1/PD-L1-IN-26 (Compound II-14) is a potent inhibitor of PD-1 / PD-L1 with an IC 50 of 0.0380 μM. PD-1/PD-L1-IN-26 activates the immune microenvironment by promoting the infiltration of CD4+ T cells into tumor tissues. PD-1/PD-L1-IN-26 has the potential for the research of cancer diseases.