Abstract: Abstract：There has been a long dispute about the age of the Bayan Obo iron-niobium-REE deposit. This paper reports the Sm-Nd isochron age and Rb-Sr isochron age of iron-niobium-REE ore samples from the west orebodies of the deposit. The Sm-Nd isochron age is 809±80 (2σ) Ma, with INd=0.511182±34 (2σ) and MSWD=1.10； the Rb-Sr isochron is approximately 391±97 (2σ) Ma, with ISr=0.70453±72 (2σ) and MSWD=47. The Sm-Nd isochron age of the west orebodies is different from the ages of the principal and east orebodies of the deposit obtained with the same chronological method. The Bayan Obo deposit may have formed in the Meso- and Neoproterozoic and undergone subsequent modification of geological processes in the Early Paleozoic Caledonian. As is the case with the principal and east orebodies, the Sr isotope system of iron ore and dolomite of the east orebodies was disturbed or reset in the Caledonian.