The objective of this study is designed to clarify the effect of tilmicosin and thyme oil on treatment of chronic respiratory disease in broiler chickens. Eighty one-day old chicks tested negative Mycoplasma gallisepticum (MG). On the 22^th day of age the chicks were divided into five equal groups Gp. (1) non infected non treated ( control ),  but chicks in Gp. (2, 3, 4, and 5) at22^th day of age were inoculated with 0.2ml of MG broth into air sac. Gp (1) non infected non treated ( control), Gp.(2) infected non treated, Gp.(3)infected treated with thyme oil 200 ppm for 5 successive days, Gp.(4) infected treated with tilmicosin 25 mg⁄kg for 5 successive days. andGp.(5) infected treated with both thyme oil and tilmicosin in same dose and period. At 1^st and 7^th day post treatment all chicks in all groups were weighed and blood samples were collected for hematobiochemical studies, biochemical studies and phagocytic activity. The clinical signs observed ininfected broilers chickens were poor appetite, depression, nasal discharge, mild conjunctivitis, sneezing, coughing, gasping, rales and mortalities were 20%.  Our results revealed that infected non treated chicks showed significant decrease in body weight gain, RBCs, Hb, PCV, WBCs, T.Protein, albumin and globuln beside significant increase in AST, ALT, creatinine, uric acid, phagocytic activity and phagocytic index at 1^st day post treatment. Treated infected broilers chickens with tilmicosin and thyme oil showed improvements in clinical signs, mortalities and body weight beside improvement in hematobiochemical and biochemical parameters and phagocytic activity post treatment. The pathological examination of internal organs of MG infected broiler chickens showed severe pathological lesions in lung, liver, kidney and heart. But infected broiler chickens treated with both thyme oil and tilmicosin is induced marked improvement in the pathological changes.  It could be concluded that the concurrent use of both thyme oil and tilmicosin is a useful mean to treat and control CRD in broiler chicks.  Thyme oil can be used as an assistant factor to retard CRD in broiler chicks due to its immune enhancer and growth promoter properties.