Purpose: To compare the dosimetric quality assessments of in- tensity-modulated radiation therapy plans generated by 6 MV and 15 MV photon energies for prostate cancer. Materials and Methods: We retrospectively analyzed the data from 30 patients treated for localized prostate cancer using IMRT to 66 Gy in 33 fractions between May 2008 and March 2010. For inverse IMRT treatment planning, we used a 7-coplanar nonopposed beam arrangement angles. To evaluate that the dosimetric quality assessment among plans are due only to energy selection, the beam arrangement, number of beam, and dose constraints were kept constant for all plans. The plan quality was determined by the measures of coverage, homogeneity, and conformity. Tumor coverage was measured using the planning target volume (PTV) and the clinical target volume (CTV) receiving 95%, 100% and 105% of the prescribed dose. Homogeneity was measured using the sigma index of the PTV and CTV. Conformity was measured using the lesion coverage factor, healthy tissue conformity index, and the conformity number. Results: Our results show a comparable coverage of PTV and CTV for both energies. The mean volume of PTV V95% was 95.0±0.3% and 95.6±0.2% for the 6 MV and the 15 MV plans, respectively. For homogeneity between 6 MV and 15 MV plan, the mean PTV sigma index and the mean CTV sigma index was 2.21 Gy (95% confidence interval) and 2.54 Gy, and 1.08 Gy and 1.21Gy. The mean conformity number was 0.84 and 0.79, respectively. Conclusion: Our study has measures of coverage, homogeneity, and conformity for IMRT plans of prostate cancer using 6MV and 15 MV photon beam. We show that the differences in PTV and CTV coverage seen between 6 MV and 15 MV plans are not very significant. However, the homogeneity and conformity for 6 MV IMRT plans is better than 15 MV.