The relationship between the propagation loss and roughness on multi-layered Si waveguides fabricated up to a 3 rd layer was investigated. By reducing the surface and sidewall roughness of the waveguides, a low propagation loss of 3.7 dB/cm for the 3 layer a-Si:H waveguides was demonstrated. Furthermore, vertical coupling between multilayer waveguides was demonstrated by use of a grating-type vertical coupler. A coupling efficiency of 22% was obtained for 10 pairs of gratings with a period of 640 nm, even with a layer distance of 1µm.