The influence of the pore shapes on the band structures in phononic crystals with periodic distributed void pores are investigated in this paper. By using finite difference time domain (FDTD) scheme, the dispersion properties of the in-plane x-y mode waves in the materials with triangular, circular or square pores are discussed respectively. The influence of the pore shapes and the porosity on the band gap structures is analyzed. The results show that for x-y mode waves, the stop bands are easily formed in the materials with triangular pores, but hard for square ones. Moreover, a critical porosity exists for the formation of the absolute band gaps. Along with the increase of the porosity, the width of the absolute band gap is increased, but the centre frequency is dropped.