Since the distribution transformers are widely used in industry, many variables, including materials, methods, and manufacturing processes directly affect its lifetime. The dry-type transformers are usually used in inner applications while heat dissipation directly influences the lifetime of a transformer, a significant improvement in this area, will result in further design like using the dry-type transformer in a closed box for outdoor applications. The reduction of the internal temperature of DTT also will conclude the reduction of its size. According to insulation properties of graphene sheet, the contribution of insulation of the winding and core is examined. Since the dry-type transformer uses standard oil-immersed paper insulation, the heat dissipation performance could be improved by addition of graphene sheet material. The results of this study showed that the graphene sheet provided a better heat transfer performance than the standard insulation, with decreases of 13-27% in the winding temperature and 24-37% in the core temperature.