To explore the correlation of radiation exposure of the periprostatic area (PPA) with erectile dysfunction (ED) after seed implant brachytherapy (BT), we compared DVH parameters of the PPA obtained from the postplan, considering alterations of the International Index of Erectile Function 6 questionnaire score (IIEF-6) after BT, among 34 patients with IIEF-6 ≧ 22 before BT. We delineated the PPA manually as the extracapsular rind of 3 mm width, which was divided into 36 parts on 12 sectors of the apex, middle and base of the prostate (ROIs). We divided patients into two groups: the non-ED group (n = 23) with IIEF-6≥22 at 12 months after BT, and the ED group (n = 11), who had IIEF - 6 ≤11 at the same time point as the non-ED group. We performed statistical comparison between the non-ED group and ED group for the mean D90 of each ROI (roiD90) and the variables belonging to patients’ backgrounds and DVH parameters. In known variables, there was no significant difference between the ED and non-ED groups in terms of patients’ backgrounds and DVH parameters. The mean roiD90 of the ED group was significantly larger than that of the non-ED group at the middle and apex of left side of the prostate. These findings indicate that that excessive doses on a specific site of the PPA may induce ED due to radiation damage on the pelvic splanchnic nerve passing through the area.