Purpose: There are differences between Standard Uptake Value (SUV) of CT attenuation corrected PET and that of 137Cs. Since various causes lead to difference of SUV, it is important to know what is the cause of these difference. Since only the X-ray CT and 137Cs transmission data are used for the attenuation correction, in Philips GEMINI PET/CT scanner, proper transformation of these data into usable attenuation coefficients for 511 keV photon has to be ascertained. The aim of this study was to evaluate the accuracy in the CT measurement and compare the CT and 137Cs-based attenuation correction in this scanner. Methods: For all the experiments, CT was set to 40 keV (120 kVp) and 50 mAs. To evaluate the accuracy of the CT measurement, CT performance phantom was scanned and Hounsfield units (HU) for those regions were compared to the true values. For the comparison of CT and 137Cs-based attenuation corrections, transmission scans of the elliptical lung-spine-body phantom and electron density CT phantom composed of various components, such as water, bone, brain and adipose, were performed using CT and 137Cs. Transformed attenuation coefficients from these data were compared to each other and true 511 keV attenuation coefficient acquired using 68Ge and ECAT EXACT 47 scanner. In addition, CT and 137Cs-derived attenuation coefficients and SUV values for 18F-FDG measured from the regions with normal and pathological uptake in patients' data were also compared. Results: HU of all the regions in CT performance phantom measured using GEMINI PET/CT were equivalent to the known true values. CT based attenuation coefficients were lower than those of 68Ge about 10% in bony region of NEMA ECT phantom. Attenuation coefficients derived from 137Cs data was slightly higher than those from CT data also in the images of electron density CT phantom and patients' body with electron density. However, the SUV values in attenuation corrected images using 137Cs were lower than images corrected using CT. Percent difference between SUV values was about 15%. Conclusion: Although the HU measured using this scanner was accurate, accuracy in the conversion from CT data into the 511 keV attenuation coefficients was limited in the bony region. Discrepancy in the transformed attenuation coefficients and SUV values between CT and 137Cs-based data shown in this study suggests that further optimization of various parameters in data acquisition and processing would be necessary for this scanner.(Korean J Nucl Med 39(3):182-190, 2005)