A new impact fusion concept is presented. Both the projectile (macron) and the fusion fuel are carefully chosen, so that the acceleration of the macron is much easier and the stopping of the fusion alpha particle is much larger. The physics of the impact process is discussed. Other than formerly believed shock compression or global adiabatic compression, we tried a fluidal plus particle diffusion approach to model the impact process. A one-dimensional code is implemented. The simulation shows, with the ion–electron temperature relaxation and bremsstrahlung being considered, that the ignition is much harder than previously believed, 1–2 MJ for a millimetre size diamond macron. Diamond DT methane impact fusion is more similar to volume ignition fusion.