Contact lengths of sanding belts for Pinus sylvestris wood and medium density fiberboard（MDF）samples were measured during sanding process by grindable bipolar thermocouple.It was found that the actual contact lengths were longer than geometric ones,and that the contact lengths decreased linearly with increase of sanding belt speeds,but increased with increase of feed rates and sanding thickness.The contact lengths of Pinus sylvestris wood was smaller than that of MDF with greater normal sanding forces.It proved that material properties except normal forces had an important impact on the contact lengths.No regular changes of the contact lengths under sanding heat generated by instantaneous low temperature were observed.