A new fabrication method of porous ceramics using unidirectional solidification was proposed in this paper and the role of silica additive on the formation of porous alumina structure was discussed. A porous alumina was formed by unidirectional solidification process that was conducted using floating zone melt method in hydrogen atmosphere at normal pressure. The effect of silica additive on the porosity and the pore size of the solidified samples was examined. The porosity of the solidified samples increased with increase of silica contents. On the other hand, the pore size decreased with increase of silica content. The average pore size of 10% and 20% silica added samples were 0.396 mm and 0.184 mm, respectively.