We present the fabrication of 150 nm half-pitch Si grating templates by reactive ion etch (RIE), which are used in nanoimprint lithography (NIL) for high groove density gratings in SU-8 plastic. The etch properties such as the etch rate, profile and etching selectivity of Si over Cr as etch mask were carefully studied. Under the optimum condition Si gratings with 150 nm in linewidth, 480 nm in height and nearly 90 degree in verticality of the sidewall have been achieved. 150 nm half-pitch gratings on SU-8 were then successfully imprinted using the fabricated templates. The diffraction pattern of zeroth and first order from the SU-8 gratings was observed using a 266 nm laser beam. The developed nanofabrication technique in this work is applicable not only for templates but also for other nanostructures in silicon.