Dextran graft poly (N-methacryloylglycylglycine) copolymer–tyrosine conjugates (dextran-g-PMAGGCONHTyr) were synthesized and characterized. Dynamic light scattering (DLS) results indicated that the graft copolymers are soluble in pH 7.4 PBS and 0.9% saline solutions. The graft copolymers were labeled with ¹²⁵I, and the labeling stability in 0.9% saline solution was investigated. Pharmacokinetics studies showed a rapid clearance of ¹²⁵I-labeled graft copolymers from the blood pool. Biodistribution images confirmed the preferable liver and spleen accumulation within 1 h after injection and rapid clearance from all the organs over time. The graft copolymer with molecular weight of 9.8 kDa was eliminated from the kidney significantly faster than those with higher molecular weight. The effect of the numbers of −COOH groups on the graft copolymers on the biodistribution was also investigated. It was found that the graft copolymers with the average number of −COOH groups per glucopyranose unit (DS_–COOH) of 0.57 and 0.18 are mainly distributed in liver and spleen at 1 h after injection, whereas the graft copolymer with DS_–COOH of 0.07 is mainly accumulated in kidney.