Fennel (Foeniculum vulgare) is a medical plant with a high content of polyphenol, it has many antioxidant properties, also it has different therapeutic effects. The aim of the present study is to evaluate the efficacy of the fennel essential oil (FEO) on some different physiological parameters in rats injected with Phenyl-hydrazine (PHZ). Rats were divided into five groups (six rats each); control group (normal rats), FEO group (administrated daily by an oral dose of FEO, 0.5mL/kg b.w. for 14 days), PHZ group (injected intraperitoneally with a daily dose of 60 mg/kg b.w. PHZ, for 3 consecutive days), protective group (administrated daily by FEO for 11 days then injected with PHZ for 3 days) and treated group (injected with PHZ for 3 days then administered by FEO for 11 days). At the end of the experiment, blood samples were collected for determination of physiological and biochemical parameters. The results showed that the PHZ induced significant decreases in rats body weight, body organ weights, serum proteins (total protein, albumin and globulin), blood glucose level and HDL –cholesterol. Administration of FEO improved body weight, body organ weights, serum proteins, blood glucose level and lipid profile in rats injected with PHZ. In conclusion, the results of the present study revealed that the FEO has improving effects on body and organ weight.