According to field investigations and magnetostratigraphic studies of the Xiyu conglomerates of the Xiyu Formation and K-Ar age determination of andesitic basalt in a correlative horizon of the Xiyu conglomerates, it is shown that the upper member of the Xiyu Formation records the lower part of the Matuyama reversed epoch (M,2. 5-1. 6Ma) and that the upper age limit is defined at about the horizon of the Olduvai normal event (O,1. 67-1. 87Ma). The isotopic age of the volcanic rocks lying between the Xiyu conglomerates and the overlying conglomerate of the high terrace is 1. 4-1. 6Ma. The lower member of the Xiyu Formation records the Gauss normal epoch (G,3. 4-2. 5Ma). On the basis of the above studies combined with the stratigraphic contact relations, biostratigraphic study and characteristics of the tectonic deformation, the lower limit of the Quaternary in Tarim is temporarily defined at 1. 8Ma B. P. (which is consistent with the international opinion). Furthermore, the conglomerates of the upper and lower members of the Xiyu Formation are assigned to the upper and lower parts of the Upper Pliocene (N22x2 and N22x1) .respectively. The determination of the age of the Xiyu conglomerates and the stratigraphic correlation of the Quaternary of the Tarim basin may guide the oil-gas exploration of Tarim,so have certain practical significance.