The aim of this study was to investigate the underlying mechanism of androgen deficiency inducing corporal fibrosis, thereby causing erectile dysfunction (ED). Forty 12-week-old healthy male rats were divided randomly into four groups: normal control group (Control); castration group (Castration); the other 20 rats were castrated followed by testosterone (T) (orally) each day: castration + 10mg/kg T group (Castration + 10T) and castration + 20 mg/kg T group (Castration + 20T). After 8 weeks' treatment, the main outcome measures were the following: serum levels of T; the ratios of intracavernous pressure (ICP) to mean arterial pressure (MAP); histologic changes in penile smooth muscle cells; the Smad and non-Smad pathways; and extracellular matrix (ECM) protein deposition. Castration group showed lower level of T and ratio of ICP/MAP, reduced ratio of penile smooth muscle cells/collagen, increased extracellular matrix protein deposition, and a higher expression of the Smad and non-Smad pathways. Castration + 10T partially preserved erectile function and histology stabilisation. However, the Castration + 20T group showed significantly better erectile function and molecular changes. Better efficacy could be expected with ART of adequate dose. Androgen deficiency induces corporal fibrosis through activation of the Smad and non-Smad pathways, and accumulation of ECM proteins.