Abstract: The mass spectrometric determination of magnesium isotopic ratios by the use of uranyl nitrate added to magnesium samples to act as a binding agent is reported. Prebaking empty filaments and preheating filaments with deposited magnesium samples on its surface in a vacuum are employed to reduce the Na signal from the rhenium-ribbon. Methods for correcting magnesium isotopic ratios for electron multiplier discrimination and mass fractionation are described in detail.The results of the determination of natural magnesium isotopic ratios are 26Mg/24Mg=0.12660(1±0.10%) and 26Mg/24Mg = 0.13938( 1±0.10%). The magnesium isotopic ratios of rich-26Mg-2 and rich-25Mg-l are 24Mg/26Mg = 0.003463 (1±0.2%), 25Mg/26Mg =0.001656 (1±0.2%) and 24Mg/25Mg =0 .006716 ( 1±0.2%), 26Mg/25Mg = 0.007264 (1±0.2%) respectively.