The present study was conducted to assess the accuracy of 177Lu quantification using Monte Carlo N-Particle Transport Code, Version 5 MCNP5. The developed code was verified against calibration factor (CF) measured experimentally. The CF for converting SPECT data into units of activity concentration was determined by modeling two phantom configurations: (1) a uniform 177Lu concentration of 5.3600 ± 0.0005 MBq/mL in 20 mL Petri dish, resulting in a CF1 of 12.5 ± 1.5 cps/MBq, and (2) a 65.4 mL radioactive sphere (5.0 cm diameter) within a non-radioactive background in a cylindrical Jaszczak phantom, yielding a CF2 of 16.0 ± 2.0 cps/MBq. The significant difference between CF1 and CF2 (21.26 %) highlights the impact of phantom size and geometry on the calibration process. The quantification error was evaluated using recovery coefficient (RC) of the spherical inserts in a NEMA phantom. The established CFs and RCs provide a reliable framework for accurate activity quantification in 177Lu SPECT imaging using the established MCNP5 code. Our findings suggest that MCNP5 simulations can effectively model the SPECT imaging process, accounting for factors such as photon attenuation and scatter, offering the potential for improved dosimetry calculations in radionuclide therapy.