Si-nanowire waveguides are attractive structures for constructing various optical devices that are extremely small, and that can be flexibly connected as devices on a silicon (Si) wafer. The waveguides can be bent with extremely small curvatures of less than a few micrometers of bending radius because the large difference of refractive indices between the Si-core (n = 3.5) and the silica cladding material (n = 1.5) strongly confines the optical power in the waveguide core. Therefore, these waveguides are promising for flexible optical interconnections on Si chips as well as for constructing many extremely small optical devices. We used Si-nanowire waveguides to fabricate optical directional couplers and demonstrated their fundamental characteristics. Their coupling-length was extremely short, several micrometers, because of strong optical coupling between the waveguide cores. Therefore, we could construct ultra compact power combiners/dividers. We have also demonstrated wavelength demultiplexing functions for these devices with a long coupled waveguide. Optical outputs from a device with a 100-μm long coupled waveguide changed reciprocally with 20-nm wavelength spacing between the parallel and cross ports. Ultra small optical add-drop multiplexers (OADMs) with Bragg grating reflectors made of the Si-nanowire waveguides have been also demonstrated. The dropping wavelength bandwidth of the OADMs was less than 0.7 nm, and these dropping wavelengths could be precisely designed by adjusting the grating period, and this indicated the possibility of applying these devices in dense WDM systems. Using the Si-nanowire waveguide, we have also demonstrated nonlinear-optic effects such as the spectral broadening of optical short pulses due to self-phase modulation and nonlinear transmittance based on two-photon absorption. At a 12 W input power level, we observed a 1.5-π nonlinear phase shift as well as strong saturation of optical output power from a 4-mm Si-nanowire waveguide sample, and that indicates the possibility of constructing integrated nonlinear-optic wavelength conversion devices or optical limiters with Si-nanowire waveguides.