Particulate matter (PM) can exacerbate allergic airway diseases. Health effects of PM with a diameter of less than 100 nm, called nano particles, have been focused. We have recently demonstrated that carbon nano particles (14, 56 nm) exaggerate allergic airway inflammation in mice. In the present study, we investigated the effects of repeated pulmonary exposure to carbon nano particles on the expression of a variety of cytokines in the absence or presence of allergen in mice. ICR mice were divided into six experimental groups. Vehicle, two sizes of carbon nano particles, ovalbumin (OVA), and OVA + nano particles were administered intratracheally. Nano particles increased the lung protein levels of thymus and activation-regulated chemokine (TARC), macrophage inflammatory protein (MIP)-1alpha, and granulocyte-macrophage colony-stimulating factor (GM-CSF) in the absence or presence of allergen. The enhancement was more prominent with 14 nm of nano particles than with 56 nm of nano particles in overall trend. 14 nm nano particle exposure significantly enhanced the lung expressions of interleukin (IL)-2 and IL-10 in the presence of allergen as compared with allergen exposure. These results suggest that pulmonary exposure to nano particles can induce the lung expression of TARC, MIP-1alpha, GM-CSF in the absence of allergen and can enhance that of TARC, MIP-1alpha, GM-CSF, IL-2, and IL-10 in the presence of allergen. The enhancing effects are more prominent with smaller particles.