The deformation and spreading of fully molten particles impacting onto a rough surface have been investigated by numerical simulation. A numerical technique, based on finite element analysis, was developed specifically for this simulation. The Lagrangian method with an automatic remeshing technique has been used to trace accurately the free surface of the molten matter and to improve the accuracy of the computation. A friction limiting condition at the particle substrate interface was introduced to describe the effects of the substrate surface roughness. This surface characteristic significantly influences the flattening degree, the flattening time, the spreading velocity of the liquid particle and its final shape.