A high-performance silicon arrayed-waveguide grating (AWG) with 0.4-nm channel spacing for dense wavelength-division multiplexing (DWDM) systems is designed and realized successfully. The device design involves broadening the arrayed waveguides far beyond the singlemode regime, which minimizes random phase errors and propagation loss without requiring any additional fabrication steps. To further enhance performance, Euler-bends have been incorporated into the arrayed waveguides to reduce the device’s physical footprint and suppress the excitation of higher-modes. Additionally, shallowly-etched transition regions are introduced at the junctions between the free-propagation regions and the arrayed waveguides to minimize mode mismatch losses. As an example, a 32×32 AWG (de)multiplexer with a compact size of 900×2200 μm2 is designed and demonstrated with a narrow channel spacing of 0.4 nm by utilizing 220-nm-thick silicon photonic waveguides. The measured excess loss for the central channel is approximately 0.65 dB, the channel non-uniformity is around 2.5 dB, while the adjacent-channel crosstalk of the central output port is −21.4 dB. To the best knowledge, this AWG (de)multiplexer is the best one among silicon-based implementations currently available, offering both dense channel spacing and a large number of channels.