For the first time, a series of Gd(AA)(3)/NR (natural rubber) composites for X-ray shielding were prepared by an in situ reaction method. Occurrence of the in situ polymerization of Gd(AA)(3) in composites during vulcanization of NR with peroxide greatly improves the dispersion level of the shielding phase by the remarkable reduction of Gd(AA)(3) particle size and the formation of small sized poly-Gd(AA)(3) from the matrix. As expected and assumed, the X-ray shielding properties of all composites apparently increase with the increase of the degree of dispersion of Gd(AA)(3) in composites.