The large refractive index difference between Si and SiO2 makes it possible to realize ultrasmall photonic integrated circuits. A 5 × 5 ultracompact arrayed waveguide grating multiplexer based on 500×250 nm Si nanowire waveguides is designed and fabricated by using the technologies of E-beam writing and amorphous-Si deposition. The measured channel spacing is about 1.5 nm (close to the design value) and the channel crosstalk is about -8 dB.