One of the risks of transperineal interstitial prostate brachytherapy is seed migration after treatment. Sometimes, seed migration cause serious side effects such as seed embolization which may cause organ ischemia. The purpose of this study is to evaluate the seed migration after brachytherapy at our facility and report on finding new predictors of seed migration. From May 2006 to April 2016, 320 patients were treated with permanent I-125 seed implantation, 202 patients were treated with brachytherapy alone and 118 patients were treated with brachytherapy combined with external beam radiotherapy. We used free seeds in all cases. We planned the prescription dose of brachytherapy alone was 145 Gy and that of brachytherapy combined with external beam radiotherapy was 104 Gy and 40 Gy (20 fraction / 4 weeks). The average age of patients was 66.2 years old (median 66 y, range 50 – 78 y). The average volume of prostate was 25.4 cc (median 24.5 cc, range 11.3 – 54.5 cc). The chest, abdominal, and pelvic radiographs were performed at 1st and 30th days after implantation (Day 1 and Day 30), and also pelvic CT was performed at Day 1 and Day 30 in each case. We investigated not only the location, the number, and the frequency of the migration, but as a new indicator of seed migration, we calculated “seed density” that was defined by the following formula: seed density = the number of Implanted seeds / prostate volume (cc). Seed migration was observed in 92 cases (28.8%) in 320 cases. The number of migration seeds was 144 out of 21876 (0.66%). In 16 cases (5.0%), seed migration was observed on day 1, and further seed dislocation was observed at Day 30. The numbers of migrated seeds located at lung region, abdominal/pelvic region, and other regions were 47 (32.6%), 89 (61.8%), and 8 (5.6%). Other sites included thigh, obturator muscle, spinal canal, kidney, and atrium. The average seed density was 2.8 (median 2.7, range 1.5 – 4.8). There was no significant difference in migration ratio by comparison for every 50 cases ( #1 – 50: 26% (13 of 50 cases), #51 – 100: 38% (19 of 50 cases), #101 – 150: 22% (11 of 50 cases), #151 – 200: 28% (14 of 50 cases), #201 – 251: 30% (15 of 50 cases), #251 – 300: 34% (17 of 50 cases), #301 – 320: 15% (3 of 20 cases)). And, there was no significant difference in migration ratio between high prostate volume group (≥ 25 cc, 34.0% (52 of 153 cases)) and low prostate volume group (< 25 cc, 24% (40 of 167 cases)). The migration ratio of high seed density group (≥ 3, 41.6% (37 of 89 cases)) was significantly higher than that of low seed density group (< 3, 23.8% (55 of 231 cases)) (p= 0.0073). The seed migration ratio after prostate brachytherapy at our institution was 28.8% (92 of 320 cases), which was comparable to previous reports. The migration ratio of high seed density group (density ≥ 3) was significantly higher than that of low seed density group (density < 3). This study showed that the seed density could be a new indicator of seed migration after prostate brachytherapy.