Introduction As the performance of a PET scanner depends not only on the scintillating material but also on the scanner design. [1] , researchers use the Monte Carlo simulation in designing the scanners or evaluating their performance and optimizing the functional parameters before commercialization. In this study, was simulated a clinical PET/CT scanner with LSO crystal using Monte Carlo method for the purpose of investigation of the performance and obtaining the optimal patient injected activity. Materials and methods The PET scanner of the Siemens Biograph6 PET/CT system was simulated using GATE Monte Carlo simulation code [2] . Sensitivity, scatter fraction (SF) and the noise equivalent count rate (NECR) metric were derived by the simulation of the standard sensitivity and count rate phantom in accordance with the NEMA NU 2-2001 protocol. GATE results were compared and validated against the published data in accordance with the vendor’s specifications. Results The simulated NEMA standard phantoms showed scatter fraction and sensitivity of 36%, and 4.16 cps/kBq respectively. The difference between the measured and simulated sensitivity of system was less than 1%, and scatter fraction was within the accepted range. Also maximum NECR value in 35 kBq/cc activity concentration was 92 kcps results in less than 5% difference between simulated and measured data. Conclusion Our results demonstrated an excellent agreement between GATE simulated data and measured data. The evaluation of the PET scanner performance in the Siemens Biograph6 PET/CT scanner using Monte Carlo modeling shows that GATE is adequately accurate to describe the performance of PET system.