The effects of grain-size in La0.7Ca0.3MnO3-δ (LCMO) system have been studied in connection with the magnetic and electronic properties. LCMO system prepared by the solid-state reaction was annealed in air at 1200, 1300 and 1400°C. The grain sizes of LCMO samples become larger with increasing of annealing temperature (TA ). The magnetization in LCMO samples increased while the coercive field decreased with increasing TA. The conductivity increased and the metal-insulator transition temperature TM-I decreased with increasing of TA. These physical properties are due to the oxygen deficiency caused by the increase of grain size. Finally, it was found that the grain size and the mechanical connection between grains play an important role in determining the electronic and magnetic properties.