Arrayed waveguide grating (AWG) is a key device in wavelength-division multiplexing (WDM) system. Polymer arrayed waveguide grating has recently attracted much attention due to its easier fabrication and higher-denisity device integration. Sidewall roughness scattering is one of the main loss factors for polymer arrayed waveguide grating. Reduction of sidewall scattering loss plays an important role in fabricatiton of low-loss polymer arrayed waveguide grating. An efficient steam-redissolution technique is used to greatly reduce sidewall scattering loss in the polymer/Si arrayed waveguide grating. The mechanism of this technique is that saturated solvent molecules adsorb and get into sidewalls of waveguides and enhance their fluidity, and then reduce roughness of sidewalls. Smoother sidewalls are achieved and verified by a scanning electron microscopy. Both straight waveguides and arrayed waveguide grating devices are steam-redissolved and reduction of their sidewall scattering loss is further measured. The sidewall loss in straight polymer waveguide is decreased by 2.1 dB/cm, the insertion loss of an arrayed waveguide grating device is reduced by about 5.5 dB for the central channel and 6.7 dB for the edge channels, and the crosstalk is reduced by 2.5 dB after the steam-redissolution.