Aim: To compare the prostate volume, as measured by transrectal ultrasonography (TRUS) and by MRI, with that of the actual prostate volume measured after a radical prostatectomy (RRP). Materials and Methods: This prospective study included 21 patients who had undergone RRP. TRUS prostate volumes were calculated using the prolate ellipsoid volume formula, with the anteroposterior diameter measured from axial (TRUS-V1) and mid-sagittal images (TRUS-V2). Two prolate ellipsoid volumes (MRI-EV1 and MRI-EV2) were calculated from the MRI using the same method, and planimetric volume (MRI-PV). The actual prostate volume (Actual-V) was measured in a measuring jug within 1 h after RRP. Results: Mean of Actual-V was 40.3ml (21.0–82.0). In paired sample tests, the correlation coefficients (R) for all methods were over 0.8. In a Student’s t test (paired), MRI-PV (p = 0.620), MRI-EV2 (p = 0.703) and TRUS-V1 (p = 0.099) showed no significant differences compared to the Actual-V. The linear regression models of these three methods were y = 1.025x – 0.268, y = 0.946x + 2.979 and y = 1.046x + 0.381, respectively. Conclusions: Between two TRUS volumes, TRUS-V1 was shown to be superior to TRUS-V2. In MRI, MRI-EV2 was more accurate than MRI-EV1. However, MRI-PV was the most accurate method. TRUS-V1 and MRI-EV2 could be used instead of MRI-PV in general clinical settings.