The initial nitridation process of Si(100)-2 X 1 surfaces has been investigated using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). The nitridation was performed by radical nitrogen at 350 degrees C. After the radical-nitrogen exposure of about 5 L, the STM image of the surface showed an increase in the density of bright spots originating from C-type defects and the creation of dark regions due to the existence of nitrides. Therefore, we confirmed that both the nitridation and the detachment of surface atoms occur simultaneously during the radical-nitrogen exposure. Hn the STM image of a 1.25 degrees-misoriented Si surface after the nitridation, the dark regions were observed along steps edges. We deduce that the nitridation occurs preferentially at the step of Si(100)-2 x 1 surfaces.