Using hybrid density functional theory (DFT), we analyzed NO adsorption on a clean Si(100) surface and on Si(100) surfaces modified by an oxygen atom or molecule. The estimated vibrational frequencies for the N-O stretching mode of chemisorbed NO molecules on both the clean and the modified surfaces were 1500-1550 cm^-1 for the N-end-down configuration and 1610 cm^-1 for the O-end-down configuration. NO dissociated easily from the clean surface via a NO dimer with a Si-NO-NO configuration, and the activation energy for the dissociation was small. [DOI: 10.1380/ejssnt.2006.624]