Soft lining material is a soft elastic and resilient material which helps alleviate stresses from the denture bases to the underlying supporting tissues in the pa-tients. The purpose of this study was to develop the new soft lining material from natural rubber to replace commercial soft lining materials which have several disadvantages and limitations. In this study, the high-ammonium natural rubber latex was vulcanized and grafted with ethyl methacrylate by γ-irradiation to improve its quality. The grafted natural rubber latex, socalled grafted copolymer, was then studied for its ultrastructure and prepared for evaluation of its bonding ability to the denture base material and cytotoxicity by using light and electron microscopies and human gingival fibroblast co-culture method. The results from electron microscopic study showed a complete junction between the grafted copolymer and the denture base material. Microscopic investigations in cell culture, the human gingival fibroblasts demonstrated well proliferation and well attachment around and on the surface of the grafted copolymer. In conclusion, the grafted copolymer has a good bonding to the denture base material and a good cellular biocompatibility. Therefore, it would be worth to modify the grafted copolymer for using as denture soft lining materials.