This paper reports a new process for the fabrication of polymer optical waveguides using gas-assisted UV embossing. A polydimethylsiloxane (PDMS) mold with a multimode waveguide array was made by casting a pre-polymer of PDMS against a plastic master of a waveguide array. The plastic master was prepared using gas-assisted hot embossing of polycarbonate (PC) film over a Si mold with the waveguide array. In this study, well-defined patterns with fine roughness for the multimode waveguides (125 mu m x 80 mu m) were successfully fabricated. The residual layer was thin and uniform. The replication quality, surface roughness and optical property of the replicated waveguides were measured and proved satisfactory. The average propagation loss of the waveguides was 0.88 dB/cm at a 1310 nm wavelength. The results indicate that the proposed method is likely to be a promising technique for the fabrication of optical waveguides.