This study was designed to investigate the influence of in ovo injection of peppermint (Mentha piperita) essential oil (PO) on hatchability (%), antioxidant states, and some biochemical parameters of broilers at hatch. Fertilized eggs (120) were divided into six groups, the control group and other groups injected with 10, 50, 100 and 150 μL/ml of PO in saline solution on the 18^th day of incubation in an air sac. GC-MS indicated that PO contains L-menthone (32.7%), menthol (29.34%) and pulegone (9.63 %) as major components. The results revealed that injection of 10 µL/mL of PO increased the hatchability compared to other groups while the body weight of the chicks was not significantly different. Antioxidant activities of glutathione peroxidase (GPx), superoxide dismutase (SOD), and catalase (CAT) significantly increased in injected groups compared to the control group. In addition, plasma total protein and globulin levels increased while the albumin/globulin (A/G) ratio was reduced. Lipid profile indicated that plasma cholesterol and triglyceride levels were significantly decreased in all PO-treated groups. In conclusion, in ovo injection with 10 µL/mL of PO showed positive effects on hatchability (%) and the viability of post-hatch chicks, without indicated harm to the blood constituents.