Recently, aromatherapy is the fastest growing one among the complementary and alternative medicine. This study conducted to identify the effects of aroma oils on Th1/Th2 immune balance. Female ICR mice with 5 weeks old and the three aroma oils; Eucalyptus (Eucalyptus globulus), Lemon (Citrus limon), Tee tree (Melaleuca alternifolia) were used. For experiment, the total 60 mice were grouped into reference (no-treatment, 6), control (distilled water inhalation, 6), low and high dose of each aroma oils (6 groups, each 8) and inhaled every other days for 22 days. After then during 7 days, all mice did not treated to test remain effects of aroma oil. Serum was collected for 4 times from orbital plexus under anesthesia at the point of 24 hours after 3 consecutive inhalations and finally after 7 days from the last collection, and the immune cell populations in serum were analyzed with flow cytometry. The B220+ cell population was showed higher in Eucalyptus treated group (x10, 29.4±6.5%; x100, 32.4±8.7%), Lemon (x10, 25.6±7.6%; x100, 30.5±4.5%), and Tea tree (x10, 21.7%±5.4%; x100, 27.1 ± 3.6%) than reference (x10, 19.2±9.9%; x100, 19.2±9.9%) and control (x10, 14.4±4.2%; x100, 14.4±4.2%) group (p=0.0039). The CD3+ and CD8+ T cell population were slightly increased in all aroma oil treated group, but these phenomenons were statistically not significant. In this study, three aroma oils did not showed the acceptable significant effects on the observed indicators of immune systems, retrospectively. However, this study is suggested that aroma oil may act as an factor which can do a role of immune shifting from Th2 to Th1.