Heterogeneity is an important feature of malignant tumor. It could appear as diversity from genotypes to phenotypes in different patients with the same type of cancer or different parts of tumor biopsies in a single cancer patient. The heterogeneity could appear in diverse genetic backgrounds, pathological patterns, differentiation stages, genetic mutation spectrum, transcriptomics and proteomics gene expression profile, etc, which indicate the high complexity and diversity in cancer progression. Tumor heterogeneity is a great challenge of cancer treatment, and is also an important research field of tumorigenesis. In this paper, we reviewed the biological features of tumor heterogeneity and possible mechanisms of its occurrence, and gave our considerations on how to design more effective personalized treatment that aims tumor heterogeneity in the era of "precision medicine".