As negative immune regulatory molecules, the inhibitory co-stimulatory molecules programmed death 1 (PD-1)/programmed death ligand 1 (PD-L1) play important roles in the adaptive cellular immunity. PD-L1 expressed in tumor cells is involved in regulating T cell activation and differentiation and inhibiting the anti-tumor immune activity of T cell through specific binding with the receptor molecule PD-1 on T cells. Currently, a variety of agents targeting the immune checkpoints of PD-1/PD-L1 have been used in clinic, which have exhibited long-lasting effect in treatment for different types of tumors. In this review, we summed up the molecular structure, expression features, factors influencing upregulation of PD-1/PD-L1, and their roles in promoting tumor growth and escaping from immune system and tumor immunotherapy.