The Baiyun Obo ore deposit in Inner Mongolia of North China is the largest REE ore deposit ever discovered in the world,whose geology gas already been described by Bai Ge et al. The six ore sanapiles from the main orebody were isotopically analysed and an isochron age of 1.64±0.38(2σ) Ga has been obtai- ned,with the initial 143Nd/144Nd ratio (INd)being 0.51088±12(2σ),corres- ponding to eNd(t)=+6.1±2.4.The age is interpreted as the time of the empla- cement of the Nb-REE-Fe ore deposit.The positive eNd(t) value (+6.1) shows that the REE of theore deposit was derived from the depleted mantle sources. The result seems to wxclude any possibility of the mixing of sigificant crustal materials during the ore deposition.