177Lu-PSMA I&T has increasingly used for targeted radionuclide therapy of metastatic castration-resistant prostate cancer (mCRPC). Dosimetric calculation, therefore, is critical to achieve the optimal therapeutic activity while sparing side effects to the normal tissues. The purpose of this study was to determine the radiation dosimetry for metastases prostate cancer patients treated by 177Lu-PSMA I&T at King Chulalongkorn Memorial Hospital (KCMH). Whole-body planar images were acquired in eight patients (12 treatment cycles) at immediately, 4 and 24 h after 177Lu-PSMA I&T injection (range 4.36 to 8.58 GBq). Region of interests (ROIs) were manually contoured on the whole-body, liver, spleen, urinary bladder, lacrimal glands, and salivary glands (parotid and submandibular glands) in order to determine the time-integrated activity (TIA) in source organs and fitted the time-activity curves with the mono-exponential function. The S-values were extracted from the OLINDA/EXM version 2.0 calculated by the NURBS computational phantoms. These S-values were used to calculate the absorbed dose coefficient in target organs according to the Medical Internal Radiation Dose (MIRD) scheme. The absorbed doses to the red bone marrow were estimated using the planar two-compartment method described by Svensson et al by separating the high-uptake and low-uptake compartment. The spherical model was used to calculate the absorbed doses in the lacrimal glands. The results showed that mean absorbed dose coefficients to the kidneys, bone marrow, liver, urinary bladder, spleen, lacrimal glands, parotid and submandibular glands were 0.81±0.24, 0.02±0.01, 0.13±0.10, 0.27±0.25, 0.16±0.07, 3.62±1.78, 0.21±0.14, 0.09±0.07 Gy/GBq, respectively. The dose constraints for the kidneys of 23 Gy, and 2 Gy for the bone marrow were not reached in any patients. The dosimetry results in this study suggest that 177Lu-PSMA I&T treatment with higher activities and more cycles is possible without the risk of damaging the organ-at-risk in prostate cancer patients.