Polyunsaturated fatty acids (PUFAs) are important nutrients because they compose biological structures and produce biologically active substances. Especially, n-6 and n-3 PUFAs are the dominant PUFAs in the brain and are involved in brain functions. It is known that not only the amount but also the ratio of n-6 and n-3 PUFAs are important for various biological functions explained by their mutual competition for metabolism, transport and acylation into cell membranes. In the modern society, intake of n-6 PUFAs has dramatically increased mainly due to the intake of seed oils. Actually, the ratio of n-6/n-3 PUFAs in the diet has steadily increased to 10-25 in industrialized countries, although appropriate n-6/n-3 PUFA ratio in nutrition is considered to be1-4. Here, we investigated effects of n-6 excess/n-3 deficient diet on neocortical development. We fed control or n-6 excess/n-3 deficient diet to pregnant mice and examined neocortical multilayer formation of their pups at postnatal day 10. Pups in the n-6 excess/n-3 deficient condition exhibited decrease in thickness of both deep and upper layers of the neocortex. We further observed impaired neuronal differentiation of neural stem cells from excess/n-3 deficient embryos. We further performed mediator lipidomics analyses and found that epoxy metabolites from n-6 PUFAs and n-3 PUFAs were increased and decreased, respectively, in the fetal brain. These results demonstrate that maternal intake of n-6 excess/n-3 deficient diet during gestation and lactation periods alters embryonic cortical neurogenesis possibly through epoxy metabolites.