Abstract: With the aim to improve the measuring precision of a roll angle based on Moiré fringe method, the influence of inconsistent periods of two gratings on the measurement precision is analyzed. From the basic calculation formula of roll angle measurement based on the Moiré fringe method, the relation of inconsistent grating periods and measuring precision is discussed. It is pointed out that the grating score error and the unparallel planes of two gratings are the main factors to produce inconsistent grating periods, while the former is a more important reason. The experimental results show that when the ratio of equivalent periods of two gratings β and measuring range are less than 1.001 and ±15′; respectively, the measuring error caused by inconsistent periods can be controlled less than 1.6″ by choosing a grating period of 50 μm and the width ranges of Moiré fringe of 1 400-1 800 μm.