The complement system is indispensable in immune response, responsible for the wide range of immune surveillance, clearance and defense. Its activation, regulated by several crucial factors, is an important prerequisite for its role in tumor growth and anti-tumor therapy. Membrane attack complex (MAC) and anti-tumor anaphylatoxins like C5a have significant effects on promoting tumor, such as upregulation of oncogenic growth factors, activation of mitogenic signaling pathways and breakage of normal cell cycle. Complement cascades, initiated by anti-tumor antibodies, also play a pivotal role in anti-tumor therapy to suppress the tumor growth. Our review focuses on the recent progress in the understanding of complement activation and the role of it in tumor growth and anti-tumor therapy, in the context of rapid development of monoclonal antibodies and nanomaterials for cancer treatment.