Silicon nanodots with very uniform sizes have been fabricated on a Si(100) surface via a thermal nitridation followed by oxygen etching reaction. N-2 gas was exposed to a clean Si(100) surface at 800 degrees C and this surface was subsequently reacted with O-2 gas at 700 degrees C. Scanning tunneling microscope measurement of surface morphology showed that silicon nanodots with an average size of 5 nm were formed as a result of selective oxygen etching of the silicon surface. Silicon nitride islands of a few nanometers acted successfully as masks against oxygen etching at high temperatures. The number density of silicon nanodots is estimated to be 1 x 10(12)/cm(2). The lowering of nitridation temperature to 700 degrees C resulted in similar silicon nanodots with a little bit smaller sizes, indicating that silicon nitride islands formed even at this temperature can be successfully used as masks for oxygen exposure. Owing to the relatively low thermal mobility of nitrogen species, Si nanodots with uniform sizes could be obtained on the Si(100) substrate.