[en] This study investigated geometrical arrangement of RI gauge using backscattered radiation. Calculations of important parameters such as distance between the source and detectors, the number of detectors and the position of the detectors were carried out using a Monte Carlo computer code MCNP4A. Depth of soil that is meaningful in determining wet and dry density was also calculated by MCNP4A. Cf-252 neutron source and He-3 detectors were adopted for wet density measurement and Co-60 gamma source and G· M detectors were used for dry density measurement. It was shown that calculate curve of backscatter type RI gauge were well agree with experiment result. As a experimental results, designed backscatter type RI gauge is superior to transmission type RI gauge in wet density, but additional study for precision measurement in dry density were need