The cutoff properties of some lower-order modes of some nano silicon-on-insulator (SOI) waveguides with air cladding or SiO2 cladding are analyzed. Operation regimes of no mode, single mode, and two modes are obtained and demonstrated by using the cutoff wavelength and the waveguide width. The beat length, between the two modes with the same polarization in the two-mode regime, are shown as a function of the waveguide parameters. Two-mode interference and the effects of the birefringence as well as the temperature are discussed to show the potential applications of the SOI waveguides.