Introduction: Air pollution, especially particulate matter with an aerodynamic diameter less than 2.5 μm (PM2.5), has become a significant public health concern in the world. Epidemiological studies have demonstrated that PM2.5 exposure is associated with many diseases including stroke, in which intracranial atherosclerosis (ICA) is largely involved. This study investigated whether inhalation of PM2.5 caused ICA and whether omega-3 fatty acids (O3FA) attenuated the development of ICA. Methods: Sprague-Dawley rats were under filtered air (FA) or PM2.5 exposure with normal chow diet (NCD), high-cholesterol diet (HCD) with or without O3FA (5 mg/kg/day by gavage) for 6 or 12 weeks. Blood lipids were measured. ICA was determined by lumen diameter and thickness of the middle cerebral artery (MCA). Inflammatory markers, interleukin 6 (IL-6), tumor necrosis factor alpha (TNF-α), monocyte chemoattractant protein-1 (MCP-1), interferon gamma (IFN-γ), vascular cell adhesion molecule 1 (VCAM-1) and inducible nitric oxide synthase (iNOS) were assessed by RT-PCR for mRNA and Western blot for protein expression. Reactive oxygen species (ROS), malondialdehyde (MDA), superoxide dismutase (SOD) activity, NADPH oxidase (NOX) activity, mRNA levels of Nrf2, HO-1, NQO-1 and protein level of NOX subunit gp91 were determined as oxidative profiles. Results: Twelve but not 6 week exposure significantly increased TG in normal diet, while PM2.5 enhanced all lipid profiles (TG, LDL and CHO) after both 6 and 12 week exposure with HCD. PM2.5 exposure for 12 weeks significantly induced MCA narrowing and thickening, in association with the enhanced expression of inflammatory cytokines (IL-6, TNF-α, MCP-1 and IFN-γ), VCAM-1 and iNOS. PM2.5 exposure for 12 weeks significantly increased ROS and MDA level, up-regulated Nrf2, HO-1, NQO-1 mRNA levels and gp91 protein level, while decreased SOD activity especially in HCD group. O3FA significantly attenuated PM2.5 induced vascular alterations, as well as inhibiting inflammatory response and oxidative injury. Conclusion: PM2.5 exposure for 12 weeks aggravates ICA in a dietary model, which was mediated by vascular inflammation and oxidative stress. O3FA dietary supplementation prevents ICA development in cerebral vessels.