The elastic wave band structures of the one dimensional rod phononic crystal are studied by the lumped-mass method. For the infinite periodic structure, the accuracy of numerical results is influenced by the number of discrete mass. The initial and stop frequecy of the first bandgap need different number of discrete mass to achieve calculation accuracy when two materials composed phononic crystal at different volume ratios. For the finite structure, the different arrangements make different width of the attenuation area at periodic load. The width of the bangap exhibits largely when the external load acts on the matrial with lower denstiy and elastic modulus in front of the higher density and elastic mudulus material.