Abstract: Complete fracture of ore is the premise of effective mechanical crushing, the drop weight impact experiments were carried out to research the size effect on complete fracture strength of magnetite, and the time-history curves of impact force of the cubic specimens, the relations of impact strength to impact velocity with different sizes, the theoretical relation of complete fracture strength to the size were analyzed.The study showed that the change of impact force of the cubic specimens have experienced the stages of rapid ascent, stable loading, descent, rebound and oscillation, effective loading of impact force lasts for 6-8 ms; significant size effects of magnetite were shown under the drop weight impact, impact strength increases with the increase in impact velocity, but the increasing rate increases with the decrease in specimen size; complete fracture strength decreases first and then increases with the increase in specimen size, the lowest complete fracture strength of specimen size is existing.The study results have certain theoretical guiding significance for optimizing the process parameters of crushing machinery and improving the crushing efficiency of ore.