This study was carried out to determine whether oregano (Origanum onites) essential oil works as a disinfectant for hatching egg obtained from broiler breeder flock. Oregano essential oil was applied at two doses 0.55 and 0.75 mu l/cm(3) and two exposure times, 3 and 6 h. The formaldehyde treated eggs were used as positive control and untreated eggs used as negative control. After chemical analysis, the main constituents of oregano essential oil were carvacrol, linalool, para-cymene and y-terpinene. The lowest microbial counts on eggs were obtained from oregano essential oil. Microbial inhibition increased with the increasing essential oil concentrations. Essential oil exposure times had no significant effects on microbial counts. Essential oil fumigation lowered middle embryonic mortality and discarded chick rate, but increased early and late embryonic mortalities compared to formaldehyde treatment. Essential oil doses significantly affected late embryonic mortality, discarded chicks rate, contamination rate, hatchability of fertile egg, body weight at 21 and 42 days, body weight gain and total feed consumption. But, early and middle embryonic mortality were not significantly affected by treatments. These results imply that oregano essential oil had great potential for hatching egg disinfectant and it could be used as natural egg disinfectant.