The microstructural dependence of thermal conductivity of a high-alumina (ca. 70%) heat-insulating frebricks(ca. 75%porosity) was investigated under special consideration of the tailored-pore shape effects. Pores different shape could be incorporated into the insulators through pore formers : Styrofoam produces spherical pores while saw dust results in parallel plate pores. Concerning the pore-shape effectiveness of thermal insulation, the specimen with irregular plate pores showed much lower values of heat conductivity than those with spherical pores, the values being 0.31 to 0.38 at $600^{circ}C$ and 0.35 to 0.47 at 100$0^{circ}C$, respectively. On the contrary, however, other material properties such as strength and softening temeprature under load were turned out to be better in the case of the spherical pores.