The effects of pore structure on the superconducting properties of dense YBa₂Cu₃O_x ceramics were investigated. The dense samples, 6.20 g/cm³, with many of small pores were prepared using a high calcining temperature (940°C), a planetary milling for a long time (24h) and ordinary sintering at low temperature (900°C) for 60h. The dense samples with the small pores showed larger pore-surface than the porous samples with large pores. Therefore, the dense sample facilitated the diffusion of oxygen within the material during the sintered processing, and it has become the orthorhombic structure. The resistivity and the critical current density of the dense samples increased about twice of that of the normal density samples.