The coupling between a silicon nanowire (NW) and a single mode fiber (SMF) is challenging. Design and optimization of compact spot-size converters (SSCs) for silicon photonics devices are presented by using numerically efficient and rigorous full-vectorial finite-element based approaches. The multi-Poly-Silicon layers based SSCs are proposed and optimized for the quasi-TE and quasi-TM polarizations sequentially. The coupling losses can be reduced to 2.72 and 2.45 dB for the quasi-TE and quasi-TM polarizations, respectively by using an eleven Poly-Si layers based SSC. A polarizationindependent SSC is also proposed based on the phase-matched multi-Poly-Silicon-layer and lower taper waveguide for both the quasi-TE and quasi-TM polarizations. Coupling to a lensed fiber with the radius of 2 μm, the optimized polarization-independent SSC is with the coupling losses of 0.34 and 0.25 dB for the quasi-TE and quasi-TM polarizations, respectively. The on-chip integrated SSC opens up the feasibility of a low cost passive aligned fiber-pigtailed electronicphotonics integrated circuits platform.