In this study, we used dual-side nanoimprint lithography to prepare sub-wavelength periodical grating (SPG)-based wave plates on flexible substrates. Without employing any etching, we directly imprinted SPGs on both sides of a polycarbonate (PC) substrate through a one-step imprinting process. This dual-side nanoimprint lithography process dramatically decreases the typical imprinting pressure required in conventional resist-based nanoimprint lithography. With this low-cost formation of PC-based wave plates, any degree of phase retardation could be obtained merely by stacking a set of wave plates with designed depths and working wavelengths. Our SPG-based wave plates exhibited the excellent broadband antireflection properties of sub-wavelength structures possessing refractive index gradients on both sides of the PC substrates. The high flexibility, low cost, and low surface reflection of these dual-side SPG-based wave plates make them attractive alternatives to commonly used birefringent crystal-based wave plates, suggesting that this phase retardation technique has promising potential for use in the development of next-generation flexible optoelectronic devices and systems. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3581084] All rights reserved.