Submicron high-purity mullite powder was prepared by a colloidal sol-gel route. Boehmite and silica were the starting materials. 2wt% $alpha$-Al2O3 or ZrO2 was used as a seeding material. The gelled powder was calcined at 130$0^{circ}C$ for 100min and attrition milled for 3hrs. The mullite powder obtained was composed of submicrometer and uniform particles with a narrow size distribution. It was hot-pressed at 1$600^{circ}C$ for 1hr under 10MPa or was sintered at 1$650^{circ}C$ for 4hrs. The bulk densities of the products made by both processes were 3.14 and 3.12g/㎤. the mechanical, thermal and electrical properties of the sintered mullite were characterized by bending strength, thermal expansion coefficient, thermal conductivity, dielectric constant and dielectric loss, etc.