The austenite grain coarsening behaviour of V-microalloyed (MA) medium carbon steel during induction heating at high frequency is studied, by varying induction input power, and using Gleeble simulating the ultrafast heating rates and Monte Carlo simulation. A significant coarsening of austenite grains occurs in the subsurface layers despite ultrafast heating rates. A distinctive feature is, unlike plain medium carbon steel, an abrupt grain coarsening below a critical depth from the surface. Gleeble experiments suggest that an abrupt grain coarsening is due to dissolution of VC particles. The Monte Carlo simulations, in a simplified temperature and particle fraction profiles, further show that the abrupt grain coarsening in V-MA steel during induction heating can occur not only by the particle pinning but also by the thermal pinning. The activation energy for grain growth is shown to be important in determining the relative contribution to the abrupt grain coarsening. The particle pinning plays an important role in the abrupt grain coarsening because the V-MA medium carbon steel, unlike Cr-Mo steel, shows moderate activation energy for grain growth.