Background: Phytoesrogenis plant derived compounds which have an estrogenic effect. Objectives: The present study was carried out to investigate some pharmacological and biochemical effects of fennel oil on male albino rats. Materials and methods: Twelve animals were divided randomly into two groups. Group A: Control. Group B: Treated rats. The rats were given an oral dose of 1 ml/kg body weight/day fennel oil once daily for one month. At the end of the experiment, blood samples were collected for biochemical analysis. Results: The fennel oil induced highly significant decrease (p<0.01) in total cholesterol(TC), triglycerides (TG), low-density lipoprotein (LDL), very low-density lipoprotein (VLDL), LDL/high-density lipoprotein (HDL), while significant increased (p<0.05) HDL. In addition, it showed highly significant decrease (p<0.02) in FSH, testosterone and sperm count compared to normal control group. Conclusion: Fennel herb has several therapeutic benefits, but it also has negative effect on male sex hormone and sperm count. So men must reduce their usage of it.