The purpose of this study is to develop lightweight pottery using organic hollow microsphere and to evaluate the mechanical properties of lightweight pottery. Spherical pores about 100 µm were formed in body, and total porosity increased proportionally with the addition of organic hollow microspheres. Many pores were isolated so that open porosity is extremely smaller than total porosity. A small amount addition of microspheres drastically degraded bending strength of pottery with slight decrease in density, indicating pores act as fracture flaws. On the other hand, a decrease in bulk density degraded fracture toughness moderately. In a wide range of porosity, a bending strength σ and a fracture toughness K_IC of lightweight pottery could be expressed in σ = 70.4d^1.57, and K_IC = 1.27d^1.54, respectively. Here, d equals to relative density.