Gamma-ray densitometry or nucleonic gauges have been widely used in industries to improve the quality of products, optimize processes and save energy and materials. Compared with common time-consuming and expensive chemical analyses, the proposed method is relatively fast and more reliable. Density measurement is normally based on the absorption of gamma radiation as it passes through the process material. The absorption which is proportional to the changes in material density indicates product density as the measuring path is held constant. In this study, a number of Monte Carlo simulations have been performed using the MCNP-4C code to optimize the arrangement of gamma densitometer. The dimensions of the proposed system have been chosen to coincide with the industrial specimen of gamma densitometer. The geometry of source to detector in different angles was investigated and optimized angles were chosen. The simulation as well as experimentally measurements has been performed for 4 different fluids including water, gasoline and diesel engine oil used with iron and PVC pipes of 4 inches diameter. The gamma source and detector have been a ¹³⁷Cs and NaI(Tl) scintillation detector. Our experiments and simulations results show that the transmission mode present, better results than the scattering one in densitometer. The single detector response of the detector located at 180