Computed tomography (CT) examinations are an essential part of the diagnostic procedures in radiology. Multidetector CT (MDCT) is becoming more and more widespread due to advances in technology. Evaluation of radiation risk and benefit is important in all radiation diagnostic procedures. The organ and effective doses are the important quantities to assess radiation risk, but an individual CT patient dose is not possible to be measured exactly. The objective of this study is to determine organ doses and effective doses in brain, chest and abdomen protocols by Monte Carlo method using ImPACT CT Patient Dosimetry Calculator version 1.0. The patient data was collected from 64 slices GE VCT scanner of 60 cases in each examination in adult male and female. The accuracy of dose calculation was verified with thermoluminescent dosimeters (TLDs) measurement in Alderson Rando phantom. The verification of the effective dose estimated from TLD dose measurements for chest and abdomen protocol showed a good agreement within 4.3% with the Monte Carlo simulation while the brain protocol showed the difference of 20.5%. For patient data collection, the average patient scan lengths were 13.4±0.65 cm for brain, 34.3±4.99 cm for chest and 41.4±3.13 cm for whole abdomen examinations. The high organ doses in irradiated field showed 37 mGy in brain and 43 mGy in eye lenses for brain examination, the dose ranged from 12 to 19 mGy occurred in lung, breast, esophagus, adrenal gland, thymus and heart for chest examination and the dose ranged from 16 to 22 mGy occurred in colon, stomach, bladder, liver, adrenal gland, small intestine, kidney, pancreas, spleen, ovaries, uterus and prostate for whole abdomen examination. The average effective doses were 1.6±0.07 mSv, 7.2±1.03 mSv and 9.7±0.46 mSv for brain, chest and whole abdomen examination, respectively. The effective doses were about 3 to 25% lower than the ICRP recommendation. The estimated fatal cancer risks were about 1, 4 and 5 cases for 10,000 populations in brain, chest and whole abdomen examinations, respectively. The scan length is one of the variable factors that make the high organ and effective doses in CT examination. The more series of examination is another factor to increase the CT doses. Estimated organ and effective doses provide an approximate indicator of potential detriment from radiation for radiologists and physicians to use as the parameters in evaluating the frequency of scan and suitable scan length.