Background: tadalafil is PDE-5 inhibitor which helps to maintain the erection by enhancing the vasodilatory effects of endogenous nitric oxide. Tadalafil has a long half-life, which gives men the option of taking it up to 12 hours before sex. Aim: To assess the effect of daily oral tadalafil 5mg for 3 months on erectile dysfunction. Patients and Methods: A case controle study involved 30 patients ED and 30 age matched controle persons. Appropriate clinical history and relevant laboratory findings (total testosterone), answering practical questionnaires that is administered in the international Index of Erectile Function (IIEF)-5 and intracorboreal injection (ICI) of 5 to 20 mg of alprostadil to asses degree of erection. Results We found that patients had significantly (p < 0.001) lower levels of testosterone (4.7 ± 0.4 ng/dl) compared with controls (5.9 ± 0.2 ng/dl) with significant (p = 0.002) increase in the levels of testosterone after treatment (8.3 ± 1.0 ng/dl) compared with before treatment (4.7 ± 0.4 ng/dl), significant increase in degree of erection by ICI after treatment( p value < 0.001) and significant increase of IIEF-5         from 12.67 ± 4.2 to20.53 ± 2.2 after treatment ( p value < 0.001). Conclusion: Oral daily tadalafil 5mg for 3 months is considered an effective treatment for ED.