Brachytherapy is term used to describe the short distance treatments of cancer with encapsulated radioactive source. Currently, high dose rate brachytherapy with Iridium-192 is mainly used in radiation treatment. In conventional brachytherapy, C-arm and ultrasound imaging device is used to simply confirm the location of source. However, the dose distribution of radiation source in human body and location of source is necessary to be measured accurately in brachytherapy. In this study, we investigate the feasibility of applying the SPECT (Single Photon Emission Computed Tomography) system for estimating the dose distribution of after-loading brachytherapy source. The three-dimensional dose distribution of brachy-therapy source detected by SPECT system was simulated using a detection system with planar detector provided two-dimensional projection of three-dimensional configuration around the phantom. Scintillation detectors which were consisted of 0.5 × 0.5 × 5 mm3 BGO pixels with 3mm parallel lead collimator were arrayed. To construct three-dimensional image in SPECT system configuration images were taken around 360 degree. The two-dimensional projection represented the dose distribution and the location of source. However two-dimensional projection limited to confirm the location of source. Therefore, projection images were reconstructed by using filtered back-projection method. The three-dimensional reconstruction image during the brachy-therapy treatment was well matched the location of source in phantom. In addition, the dose distribution in phantom was able to be acquired through the SPECT system.