Purpose: I-124 has a long half life of 4.2 days that is suitable for imaging over several days during the biological uptake and washout of radioiodine. However 1-124 has a low positron branching ratio (23%). High-energy y- photons (602 keV to 1,326 keV) are emitted in cascade with the positrons. These cascade γ- photons degrade the image quality. In this study, noise equivalent count rate (NECR) was measured with the various energy window settings to find the optimal energy window setting in 1-124 PET on newly released Siemens Inveon PET. Sensitivity and scatter fraction (SF) on Inveon PET were also assessed with the same energy windows. In addition, NECRs of 1-124 on microPET R4 (R4) scanner were also assessed for the comparison. Methods: Monte Carlo simulation studies were performed to find the optimal energy window setting. System performance such as the sensitivity and scatter fraction was measured in both 1-124 and 18F. NEMA NU-4 rat phantom and mouse phantom were simulated. Source activity was 1 MBq to 150 MBq. NECR in I-124 PET was calculated as following equation: NECR = T2 / (T + S + 2fR + fD) Where, T, S, R, D, and f is dirty coincidence corrected true, scatter, random, dirty coincidence count rate, and average fraction of the projection taken up by the object. Simulation for NECR were repeated under four different conditions (energy window: 250-550, 250-650, 350-550 and 350-650 keV). Results and Discussion: Within an energy window of 350-550 keV, dirty coincidence fraction was dramatically reduced by 50% on Inveon. Comparing two microPET systems, Inveon has 10 times higher NECR than those of R4. This would be due to improved electronics which enable to minimize the system dead time. Considering NECR of I-124, optimal energy window was 250-650 and 350-550 keV for mouse and rat, respectively. The proposed optimal energy window setting would boost the image quality of I-124 PET.