We compared sexual function by the expanded prostate cancer index composite (sexual domains of EPIC), health-related quality of life (SF-8), and International Prostate Symptom Score (I-PSS) inpatients using tadalafil after prostate brachytherapy (PB). Forty-five patients who underwent PB between April 2011 and January 2014 were included in this study. Patients were divided into the tadalafil (20 mg,once/week or once/two weeks) treated and non-treated (NT) groups. Sexual function was assessed prior to PB treatment and followed up to 24 weeks after PB. SF-8, sexual domains of EPIC, IPSS and subjective symptoms were assessed pre-PB and at 4, 8, 16, and 24 weeks post-PB. Patients in the tadalafil group achieved higher sexual function scores compared to NT group at all time points. For SF8, the patients in the tadalafil group significantly improved in mental health by the eighth week, and significantly worsened in the NT group (8 w ; p = 0.04). The voiding domains of EPIC score were found to worsen significantly after 4 weeks from PB in both groups, but the score tended to improve over 24 weeks. There was no significant difference between two groups. The I-PSS total score was found to worsen significantly in both groups post-PB, but the tadalafil group had a tendency to worsen less. PB treatment of localized prostate cancer is preferred for the preservation of sexual function. Management of sexual dysfunction with tadalafil after PB does not worsen sexual functions. We concluded that tadalafil might be applicable to mental health care in the treatment of patients with a high interest in sexual function before PB.