Cancer is a catabolic inflammatory disease that causes patients to often experience weight loss, or even cachexia in severe cases. Undernourishment in patients with cancer impairs the quality of life and therapeutic response, further leading to poor prognosis. Active and frequent nutritional screening and assessment using valid tools are important for fast and appropriate nutritional intervention. Additionally, a suitable individualized nutritional intervention strategy should be established based on the nutritional assessment result. In general, nutritional intervention begins with nutritional counseling of patients diagnosed with cancer, and a well-planned nutritional counseling improves the treatment adherence and nutritional status. When planning nutritional supplementation for cancer patients, specific nutrients, including amino acids and fatty acids, should be considered. However, there has been no consistent result showing that any particular nutrient significantly improves the prognosis of cancer patients. Hence, continuous attention from clinical physicians is needed to plan nutritional improvement in patients with cancer.