Abstract: The gravimetric method is established for measurement of magnesium oxide in nickel ore, by which the mass fraction of magnesium oxide is calculated with the magnesium ammonium phosphate precipitation produced from ammonium dibasic phosphate. With study on the effect of nickel occurrence, temperature, precipitant, aging time and interference elements on the results, it is concluded that the optimum conditions include burning temperature of 1100℃, 20 mL of precipitant dosage and aging time more than 5 h. The method detection limit is 1%, with relative standard deviation of 0.31%-4.38% and recovery rate of 98.9% to 100.3%. The method is accurate with simple operation, suitable for determining magnesium oxide in nickel ores.