Abstract: The hypothesis of tumor stem cells believes that a small proportion of cells which have the stemness property reside in tumor. This population is the reason of tumor formation, growth, metastasis and relapse. Only the therapy targeting this population could make tumors curable. At present, most anti-cancer protocols merely kill the majority of differentiated tumor cells and hardly affect the tumor stem cells. That is why most of the therapies do not achieve good results. Disrupting the pathways and niche which regulate the tumor stem cells' self-renewal, or inducing the tumor stem cells' differentiation, or targeting the surface markers which distinguish the tumor stem cells from normal stem cells are all probable strategies.