Optical intensity distribution in photocuring nanoimprint lithography is investigated. Its dependency on pattern size, optical index, and residual thickness of the photopolymer has been numerically evaluated using the finite-difference time-domain method. The irradiated beam propagates mostly in the photopolymer, where the optical index is larger than that of the quartz mold. If the pattern size is less than around 1/4 wavelength of the irradiated beam, optical intensity in the polymer decreases because the light propagates through both the polymer and quartz molds. In addition, the index difference between mold and polymer increases diffraction into the residual layer and causes optical. intensity concentration at a particular position. (C) 2003 American Vacuum Society.