The immunomodulatory effect of peppermint and eucalyptus essential oils in broiler chickens immunocompromised by either infection or vaccination with infectious bursal disease virus (IBDV) was investigated. No significant difference in HI titers was observed in volatile oils treated group over their untreated one at any interval. However, the protection percentage reached 35% in treated group and was 25% in untreated one as compared with 0% in unvaccinated control group when challenged with velogenic viscerotropic Newcastle disease virus (vvNDV). Significant increase in Bursa / body weight ratios was observed between IBDV vaccinated treated group as compared with untreated one at 3 and 10 days post- vaccination (PV). In addition, significant increase was observed between IBDV infected treated birds versus infected untreated group at 3 days post-infection (PI). These findings indicate that treatment with volatile oil ameliorated the immunocompromised effects of IBDV vaccine or infection on immune system of treated birds. The positive effect of volatile oils on cell mediated immunity of treated birds revealed significant increase in phagocytic %, lysozyme and nitric oxide activities at 14 and 28 days of age in IBDV vaccinated and treated birds as compared with IBDV vaccinated and untreated birds. Moreover; the phagocytic index was higher in IBDV infected and treated birds over non treated ones at 42 days of age. Histomorphological examination of the major immune organs clarified lymphocytic necrosis and depletion, hyperplasia of reticuloendothelial cells, vacuolations of lymphoid follicles, interfollicular lymphocytic cells infiltration and interfollicular connective tissue proliferation of bursa of Fabricius (B.F). Lymphocytic depletion and hyperplasia of reticular cells of spleen. Lymphocytic necrosis, depletion, focal hemorrhage of thymus glands. Necrosis and depletion of caecal tonsils. The sum lesion scores were 0, 0, 2 and 1, 1, 5 in volatile oils treated and untreated chickens at 3, 7 and 10 days PV respectively. While the sum lesion scores reached 21, 23, 14 and 21, 23, 20 in volatile oils treated and untreated chickens at 3, 7 and 10 days PI respectively. On the other hand there was lymphoblast activation reached score in BF in volatile oils treated group only at 3 and 7 days PV. No histopathological changes all over the experimental period could be detected in blank control group. In conclusion our study reveals that eucalyptus and peppermint oils have a potent immunomodulatory effects and are able to evoke the immune response in IBDV vaccinated or infected chickens.