The functions of n-3 fatty acids are known to be diverse, and they play roles in cardiovascular and neuronal systems and in lipid metabolism. Docosahexaenoic acid (DHA), which is the most abundant n-3 fatty acid in the brain, is essential for the maintenance of brain functions throughout the human lifespan. Epidemiological studies have demonstrated that reduced n-3 fatty acid intake is closely associated with the onset of mental and neurological diseases such as brain developmental disorders, depression, and Alzheimer's disease. DHA is primarily involved in neurogenesis, synapse formation, neuronal differentiation, neurite outgrowth, maintenance of membrane fluidity, anti-inflammatory action, and antioxidant action. Its mechanism of action include: 1) the effects on ion channels and membrane bound receptors/enzymes achieved by changing membrane fluidity, as a cell membrane constituent, and 2) free DHA molecules, derived from the cell membrane that directly or metabolically, by conversion to protectin D1 and other molecules, indirectly regulates the gene expression and the activity of intracellular proteins. Although future studies are required, the supplementation of n-3 fatty acids such as DHA may suppress the deterioration of brain functions, delay the onset and progression of various mental/neurological diseases, and further improve the outcome of the neuronal diseases.