The pattern resolution of soft lithographic techniques is critically determined by the elastic modulus of the soft mold that can support fine and high-aspect-ratio features with conformal adhesion to target substrates. We present a strategy to fine-tune the elastic modulus of conformal molds made of polyurethane acrylate by optimizing the chemical structures and the composition of prepolymer and modulator. Trimethylolpropane ethoxylated (15) triacrylate plays a key role as a delicate modulator for increasing the elastic modulus of soft aliphatic urethane diacrylate oligomer with its low cross-linking density. The optimized molds have sufficiently high elastic modulus (>23 MPa) for defect-free replication of dense, high-aspect-ratio (>2) nanopillars and nanotrench structures while still preserving their conformality. The conformal mold with good mechanical and optical properties can serve as a semi-permanently usable optical phase mask with a wide range of phase modulations for generating three-dimensional (3D) nanostructures with high precision.