The integration ultrathin high dielectric constant (high-k) materials with graphene nanoribbons (GNRs) for top-gated transistors can push their performance limit for nanoscale electronics. Here we report the assembly of Si/HfO(2) core/shell nanowires on top of individual GNRs as the top-gates for GNR field-effect transistors with ultrathin high-k dielectrics. The Si/HfO(2) core/shell nanowires are synthesized by atomic layer deposition of the HfO(2) shell on highly doped silicon nanowires with a precise control of the dielectric thickness down to 1-2 nm. Using the core/shell nanowires as the top-gates, high-performance GNR transistors have been achieved with transconductance reaching 3.2 mS microm(-1), the highest value for GNR transistors reported to date. This method, for the first time, demonstrates the effective integration of ultrathin high-k dielectrics with graphene with precisely controlled thickness and quality, representing an important step toward high-performance graphene electronics.