Abstract: Dimension measurement is one of very important application fields of ICT detection. Compared with general method, ICT uses the pixel size and value to get the real value. But if the size of item is closed to the BW (effective beam width), half width method will cause large error and is no longer applicable. In order to solve this problem, at this paper, the MTF curve was chosen as the characteristic quantity and mathematics software was used to calculate the size of the small defects. In our experiments, through using this method we get the result that the measurement error is 13. 8% at most, whereas the error for more than 87% of measurements is less than 10%. So we can get the conclusion that this method is effective.