To describe a transrectal ultrasound (TRUS) guided intraoperative prostate seed implantation process based on preplanning, and to demonstrate its advantages in dosimetry and operating room (OR) efficiency. A new integrated TRUS (TargetScan) and treatment planning (VariSeed) system was used for image-guided intraoperative seed implantation and planning. Two to four weeks before implantation, a TRUS volume study was performed to establish the prostate volume and a preliminary seed loading in order to maintain OR efficiency. The radiation oncologist contoured the prostate and devised a preplan using modified peripheral loading. The physicist checked the preplan and ordered uniform-strength, stranded 125I seeds with 10% extra loose seeds. After aligning the prostate to the preplan images with patient at treatment position in the OR, another TRUS volume study was acquired. The intraoperative prostate and urethra were contoured. All preplanned stranded seeds were implanted while the positions of the needles and seeds were tracked and marked on real-time biplanar ultrasound images using VariSeed. The plan was reviewed and extra needles and seeds were then implanted to improve the prostate coverage. Postimplant dosimetry was obtained with a CT scan on the implantation day. We compared the dosimetry between these plans for ten consecutive patients. Since we followed the preplan and all preplanned seeds were loaded in strands, the seed implantation process took only 30 - 45 min. The setup, alignment and image transfer took ∼15 min. The review, extra seed implantation took ∼5 min. On average for preplan, intraoperative plan and post plan, respectively: Prostate volume was 31.0, 36.6, and 37.0 cc (p < 0.01 and p = 0.78); Prostate V100 were 99.3%, 99.0%, and 87.2% (p = 0.23 and p < 0.01) of the prescription dose; Prostate D90 were 115.1%, 122.0%, and 96.8% (p < 0.01 and p < 0.01); Prostate V150 were 36.1%, 50.8%, and 34.3% (p < 0.01 and p < 0.01). On average 84 seeds were required for preplan with 7 extra seeds for intraoperative plan per patient. Implanting the extra seeds improved the prostate coverage from 95.5% to 99.0% for V100, 109.0% to 122.0% for D90, while increased V150 from 33.8% to 50.8%, urethra D30 from 128.1% to 140.2% (all p < 0.01). Compared to other intraoperative prostate implantation techniques that do not use a preplan devised before the OR day, our technique has advantages of high OR efficiency, straightforward seed implantation, and no need for special implantation devices. The dosimetry analysis of ten patients showed comparable or improved results in target volume coverage and plan conformality.