The technique for the control of porosity and pore size in sintered ceramics has been developed and the effect of the controlled microstructure on mechanical properties has been measured for the model system of $Al_2O_3$. The porosity and pore size in $Al_2O_3$ could be controlled by using polymer powders of constant size of 3, 6 or 12 μm. For getting a uniform microstructure, uniform distribution of polymer powder was essential. After fast firing at 1750℃ for 30 min, the pores of approximately the size of polymer spheres were uniformly distributed in an $Al_2O_3$ matrix of about 7 μm in grain size. The fracture strength measured by 4-point bend test was not sensitive to the porosity up to 0.6 vol% but deceased slightly above 0.6vol% for specimens with pores of 3 μm in size. In contrast, the fracture toughness measured by ISB (Indentation/Strength-in-Bending) deceased considerably by adding 0.2vol% porosity but remained constant with increasing the porosity. For specimens with constant porosity of 0.5vol%, the MOR of the specimen with pores of 12 μm was the highest while the fracture toughness decreased considerably with increasing the pore size.
