In this work, nanowire forests are fabricated by using a plasma repolymerization technique. The fabrication process involves only spin-coating of polyimide and plasma bombardment of the polymer, the whole process is lithography-free, micromachining-compatible and extra simple. Besides, the nanowire forests can further be utilized as nanomasks in reactive ion etching for nanopillar-nanowire composite forests. Optical features of nanowire forests and nanopillar-nanowire composite forests in different wavelength regions are investigated. The high transraittance of nanowire forests in visible-light range implies applications of the forests in transparent devices. Composite forests with a special absorptance peak in range of 8-10 μm implies broader applications of the nanoforests in infrared devices for specific substance sensing.