We proposed an ultra-thin Si-padded Si3N4 waveguide consisting of a very thin Si slab underneath a Si3N4 strip separated by a SiO2 layer. The Si slab and the Si3N4 strip form a hybrid waveguide mode, where the mode field is confined in these two structures. The measured waveguide propagation loss is 0.055 dB/cm, and the bending loss is 0.09 dB per 90° bend, depending on the bending radius. Because part of the waveguide mode is distributed in the Si slab, this waveguide structure has potential for implementing low-loss and high-speed photonic integrated circuits.