We demonstrated a 51 cm long waveguide array on a poly(methyl methacrylate) sheet fabricated by silicon hard-molding technology. To reduce the silicon sidewall roughness, a wet-oxidation followed by a buffered oxidation etchant etching process is adopted, achieving a surface roughness of 1.2 nm. The waveguide obtained a total insertion loss of -15.1 dB and an adjacent channel cross talk below -31 dB. The 3 dB optical bandwidth is determined to be 150 GHz by the optical autocorrelation method.