Abstract: Immune evasion is an important hallmark of cancer， and a better understanding of this mechanism is essential for the development of effective strategies against cancer. The programmed cell death lagand 1（PD-L1， also know as B7-H1）／programmed cell death 1（PD-1， CD279） pathway has been demonstrated as a major mechanism of immune evasion in tumor site. In recent years， anti PD-L1／PD1 immunotherapy has achieved encouraging success. Inducible B7-H1 expression in tumor microenvironment is complex. Understanding these interactions and how tumors take advantage of this pathway can help us design future strategies for better therapeutic efficacy and to overcome drug resistance. In this paper， the expression of PD-L1 and the relevant mechanisms are summarized.