Abstract: Objective: To study the influence of different dietary magnesium on metabolism of calcium and phosphorus, evaluating effect of magnesium deficiency on nephrocalcinosis. Methods: 30 Sprague Dawley male rats were divided into three groups, 10 each and fed in non-metal metabolic cages individually. The rats in the first group were fed Mg deficient diet （86 mg Mg /kg diet）, in the second group were fed Mg adequate diet （548 mg Mg /kg diet）, while in the third group fed Mg excess diet （5402 mg Mg /kg diet）. After 25 days adaptation to pair-fed, a five-day metabolic experiment was carried on. Results: In spite of 10 times higher of magnesium levels in diets of group 3 than that of group 2, the apparent absorption of calcium in group 3 only decreased 2.45%. However, urinary Ca excretion was 3 times higher （1378.6μg/d） in group 3 compared with group 2 （427.8μg/d）, so excessive dietary magnesium decreased retention of Ca. On the other hand, kidney weight in Mg deficient rats was significantly higher. Mg deficient diet promoted the deposits of Ca and P in kidney, especiallycalcium in kidney was high up to 2.23±2.91mg,43 times higher than that in Mg sufficient group（0.052±0.006 mg）Conclusions: Mg deficiency may be one of the important pathogenic factors in nephrocalcinosis. It may be an effective nutritional intervention for prevention of nephrocalcinosis to provide with sufficient magnesium in diet.