In this letter we report the use of roughly polished aluminum substrates with spin-coated polymer-smoothing layers for the fabrication of pentacene field-effect transistors. Transistors with spin-coated poly(methylrnethacrylate) gate insulator layers were fabricated and showed good performance. On the gate insulator surface, the root-mean-square roughness was found to be 0.18 nm. significantly smaller than the aluminum surface roughness, which is on the scale of tens of nanometers. Field-effect carrier mobilities extracted from the device data reached 0.75 cm 2 V -1 s -1 ; the maximum on/off current ratio was near 05 × 10 6.