Subwavelength periodic gratings, which are widely used in optical systems, exhibit only zero-order diffraction, owing to a diffraction limit. This limits the resolution of optical critical dimension (OCD) metrology. We propose a method to enhance the diffraction of subwavelength periodic gratings using a far-field superlens (FSL), which has the potential to obtain higher resolution in OCD metrology. All simulations in this study are performed using the rigorous coupled-wave analysis method. An FSL grating structure is developed for verifying the proposed method and for evaluating the role of the incident angle, misalignment, and air gap on the diffraction by the FSL grating structure. The simulation results show that, when overlaid with an FSL, a silicon grating with a period of 100 nm, which evoked zero-order diffraction only, can also evoke first- and second-order diffraction in the case of a large incident angle. Last, the potential application of an FSL in OCD metrology is discussed.