The densifying behavior of ultrafine amorphous Si3N4 (about 20 nm)-$eta$-SiC (about 40~80 nm) powders (O2 : 1.3~15wt%, 0<C<8 : wt%) was investigated with a particular attention to the microstructural changes. The specimens were heated to 1$700^{circ}C$ within 15 sec and then immediately cooled and held at 135$0^{circ}C$ for 10 min in N2 atmosphere without resorting to additives using a Xe image heating apparatus. Using an ultrafine pure Si3N4 powder with particle size less than 30nm, further more, mixed with an appropriate amount of $eta$-SiC, was found to be advantageous to obtain uniform and homogeneous microstructure. In addition, ultrafine Si3N4 powders were also proved to be effective as sintering additive on densifying large sized Si3N4 powder compacts.