Purpose: We examined and elucidated the physical mechanism of penile erection in normal and pathological conditions of vascular origin. Materials and Methods: A nonlinear lumped parameter mathematical model of penile hemodynamics was developed based on physical structure and physiological function. The model was applied to simulate the normal erectile mechanism and the pathological conditions of arterial insufficiency and venous leakage. Results: The model showed physiological behavior and suggested explanations for the relationships of corporeal pressure and venous flow limitation during erection. Other results related the reduction in rigidity and tumescence to the type and severity of vascular impairment. Conclusions: This model can be used in future studies of the diagnosis of erectile dysfunction.