This paper was researched the effect of slurry stability on the mechanical and electrical property of the porcelain insulator with various raw materials such as feldspar, quartz, clay and l7wt% alumina. The slurry was fabricated after ball milling the mixed raw materials. Green compacts were made by the extrusion and were sintered at 1300$^{C}$ for 60min in the tunnel kiln. All of the specimens were densified 96% of the theoretical density. The 3-point flexural strength($sigma$$\_$B/) of the specimen stabilized slurry pH 7.8 was 1650 k9/㎠ and the vickers hardness(Hv) and the fracture toughness(K$\_$IC/) were 27.5 GPa and 2.2 MPa$.$m$^$$sfrac{1}{2}$/, respectively. The mechanical properties of the specimen stabilized slurry PH 9.3 were 1716 kg/㎠($sigma$$\_$B/), 27.6 GPa(Hv) and 3.0 MPa$.$m$^$$sfrac{1}{2}$/(K$\_$IC/), respectively. The dielectric strength was increased from 8.3kV/mm to 13.2kV/mm as the increase of the slurry pH from 7.8 to 9.3. Therefore the physical properties of the specimen stabilized slurry pH 9.3 were improved.