A high creep resistance at specified temperature and compressive stress was obtained in this investigation in the silicon nitride/ silicon carbide composite with a nano-nano structure (nanosized SiC and Si3N4 in dual-phase mixture) by a novel synthesis method. Starting from an amorphous Si-C-N powder derived from pyrolysis of a liquid polymer precursor, nanocomposites with varied grain size were achieved. With yttria additive amount decreasing from 8 to 1 wt% and eventually to zero, the structure underwent a transition from micro-nano (nanosized SiC included in sub-micron Si3N4) to nano-nano type. Nanocrystalbne silicon nitride/silicon carbide ceramic composite with 30-50 nm grain size was synthesized without using sintering additive.