In the present study, the response of Japanese Quails to dietary Thymbra spicata L. essential oil was investigated. A total of 192 Japanese quail chicks, 1-day old and of mixed sexes, were used in the feeding trial. Birds were randomly allocated to four dietary treatments comprising three experimental groups and one control group (basal diet). Each group was divided into four subgroups, with each containing twelve chicks. EO was added daily to the basal diet at levels of 200 (T1), 400 (T2) and 600 mg/kg (T3), respectively. There was a significantly increase in live weight at 35 days when quails were supplemented with increasing level of EO. Moreover, live weight gain significantly increased over ranges of 28-35 and 0-35 days in quails fed the EO at 600 mg/kg level compared to the control group (P<0.05). No differences were observed among treatments for feed intake. A significantly improvement in feed efficiency at 28-35 days (P<0.001) was observed in treated groups. Treatments significantly decreased the number of total bacteria and Escherichia coli in the small intestine. A significantly decrease was observed for TG levels in all treatment groups. TG/TC rate was significantly decreased in supplemented groups (P<0.001). Treatments did not alter serum HDL and LDL levels. In conclusion, dietary T. spicata L. EO enhanced the performance parameters by showing time-depended effects. This property could be important to recommend the implementation time of such compounds to the poultry diets.