In order to measure the variation of displacement or gap of two parallel planes in nanometer level, a grating-based metrology was brought forward and researched. Two superposed micro-gratings with slightly different periods were adopted to generate a set of periodic moiré fringes, the period of which was hugely magnified with regard to that of two gratings. And the relative displacement of gratings was encoded in the phase of fringes. Firstly, an approximate theoretical model about distribution of complex amplitude of fringes of two gratings was built. Then, a method of measuring in-plane displacement and gap of two parallel planes was designed on the basis of this model. Finally, numerical computation about the process of movement of two gratings to generate corresponding fringes was performed. The results show that a small relative displacement of two gratings leads to a large displacement of corresponding fringes and this method can resolve the variation of displacement or gap of two parallel planes within nanometer.