We report the improvement of the hardness and modulus properties in a silsesquioxane-based soft replica mold by adding thermal initiator, without deteriorating the UV transmittance at the wavelength of 365 nm. It is found that thermal initiator used for this work contributes to improving the hardness and modulus values up to 0.175 and 3.585 GPa, while the UV transmittance value is still above 75%. The optimized soft replica mold built on a flexible plastic substrate allows submicron-scale patterns to be transferred onto a rigid Si substrate by means of UV-NIL process. Consequently, we demonstrate that the developed soft replica mold can be a suitable replacement for typical hard molds, promising further use in mold-based nanolithography for the fabrication of high-resolution nanopatterns over large areas.