Abstract: This paper describes the modification of МИ-1305 Mass Spectrometer for de-termining ~(235)U/~(238)U isotopic ratio of UF_6, which was done according to the designof CH_4/UF mass spectrometer. A molecular beam inlet system and a spherical coldtrap were used. The GKB-10diffusion pump was replaced by a mercury diffusionpump (150 1/s).An ion source modelled on CH_4/UF mass spectrometer and itspower supply were made. The vacuum is 6×10~(-8) torr.The resolving power is 310with an entrance slit of 0.1 mm in width and an exit slit of 0.35mm in widthThe memory factor is about 0.01. When used for measuring the abundance ratiof a sample pair, the difference is less than 0.001 compared with the measuringresults of other sophisticated mass spectrometers.