In this work, we have developed low-cost, high modulus, flexible, and UV transparent polyimide plastic molds for nanoimprint lithography (NIL). Different structures of poly(amic acids) (PAA) and polyimides (PI) have been synthesized. By casting the PAA or PI solutions on a silicon master, flexible but still rigid plastic molds can be produced. The advantages of the PI molds are: (1) high glass-transition temperatures (Tg) up to 310 degrees C, (2) high thermal stability over 500 degrees C, (3) high tensile modulus, and (4) UV transparency for use in UV-NIL. Various micrometer and nanometer scale patterns could be obtained from the PI molds on a large area (4 inch wafer). The imprinting results showed that the PI molds could be faithfully used for both hot embossing NIL and UV-NIL.