Background: Studies have demonstrated that plant extracts have various biological effects including immunomodulatory effect. Objective: In the present study, the immunomodulatory effects of Lavandula officinalis on lymphocyte proliferation and tumor necrosis factor alpha cytokine production were investigated. Methods: [3H] - thymidine incorporation assay on stimulation of the peripheral lymphocytes with PHA and determination of tumor necrosis factor alpha production was assayed, using LISA method. Results: The herbal extract in a concentration of 50 g/ml and lesser, in the presence of mitogen increased peripheral blood lymphocytes proliferation. However, there was not significant effect in concentrations more than 50 μg/ml. Also, the extract in the concentration of higher than 50 g/ml suppressed the production of tumor necrosis factor alpha in the exposed cells. Conclusion: The extract showed immunomodulatory effect by increasing of lymphocyte proliferation in a concentration of 50 g/ml and lesser. Also, decreasing in tumor necrosis factor alpha in the concentration of more than 50 g/ml was observed. Further studies, should focus on identifying the immunomodulatory mediator of this herbal extract.