In order to build a nano-device on polymer substrate, nano-size patterning must be done. However, conventional photolithography cannot be used to fabricate nano-sized patterns on polymer film due to the flexibility of polymer film and its potential interaction with developer solution and organic solvent. In this study, 100nm sized dense line and space patterns were made on flexible PET (polyethylene-terephthalate) substrate using newly developed monomer based imprinting lithography. Compared to hot embossing lithography, thermal curing imprint lithography uses monomer based imprint resin which consists of base monomer and thermal initiator. Since it is liquid phase and polymerization temperature is much lower than glass temperature of polymer, the nano-sized patterns can be transfer at much lower temperature and pressure. Hence, patterns as small as 100nm were successfully fabricated on flexible PET film substrate by monomer based thermal curing imprinting lithography at 85°C and 5atm.