The rapid heating of ultrafine Si3N4 powder compacts with relatively high oxygen content was investigated with particular attention to their microstructures. The specimens were heated without resorting to additives and pressure under controlled thermograms attained by an Xe image heating apparatus. The effects of particle size and oxygen contents, as well as heating conditions, were investigated. When fired to 1700 degrees C within 15 s and then immediately held at 1350 degrees C for 10 min in N-2 atmosphere, significant densification took place in the limited region, in addition to decreasing the oxygen content to less than 0.3 wt%. This decrease of oxygen content was drastic and found to be a prominent feature of this heating process; especially, weakly oxidized ultrafine powder less than 30 nm was found to be advantageous to obtain uniform and homogeneous Si3N4 microstructures.