Cancer-related fatigue (CRF) is one of the most common and complex symptoms experienced by patients with cancer. It is one of the most distressing symptoms and can diminish quality of life profoundly. This article mainly focuses on the current hypothetic mechanisms and pathophysiology of cancer-related fatigue. The proposed mechanisms include proinflammatory cytokine hypothesis, hypothalamic-pituitary-adrenal-axis dysfunction, serotonin dysregulation, circadian rhythm modulation hypothesis, anemia, adenosine triphosphate hypothesis, vagal-afferent-activation hypothesis, growth factor hypothesis and genes. These mechanisms provide a basis for future research of CRF and provide new routes for clinicians to develop diagnosis, prevention and further treatment.