We evaluated dose calculation accuracy for modi- fied Cone-Beam computed tomography (CBCT) images using Multi -level-threshold algorithm, and compared it with un- modified CBCT images. Inhomogeneous pelvis phantom imag- es and three prostate cancer patient images were acquired from both planning CT (PCT) and CBCT. The Hounsfield Units (HUs) were measured for air, tissue, bone. Using in- house software, three different tissue types were differentiated in both imaging modalities. The HUs of CBCT images were replaced by the mean HUs of the same tissue type of the PCT. An intensity modulated radiation therapy (IMRT) plan was created on PCT and copied to the modified CBCT. To evaluate the dosimetric accuracy, dose distributions based on CBCT images were compared it with PCT for four datasets (one inhomogeneous phantom, three prostate cancer patients ). HU- ED calibration acquired with PCT was used in both images. In the inhomogeneous pelvis phantom, differences in dose param- eter were seen. Regarding PTV, Rectum, and Bladder, differ- ences in mean dose between PCT and CBCT were 0.4%, 0.4%, and 0.1%, respectively, whereas those between PCT and modi- fied CBCT were 0.2%, 0.1%, and 0%, respectively. In the patient study, the average dose difference of PTV, Rectum, and Bladder between PCT and CBCT were 1.0%, 0.2%, and 0.6%, respectively, compared to PCT and modified CBCT, 0.5%, 0.1%, and 0.2%, respectively. We found that the dose difference in the plans based on PCT and CBCT were de- creased after this modification. The uncertainty in patient positioning between PCT and CBCT may cause additional discrepancies in the calculated results compared to the results in the phantom study. An accurate dose calculation based on CBCT images is possible when density distributions are cor- rected. This method does not need to acquire HU-ED calibra- tion for CBCT, and may be able to remove scatter.