Abstract: Duplex wire image quality indicator (IQI) is often used to measure unsharpness in radiography, or to determine basic resolution of an X-ray digital detector. However, manual determination of modulation of the IQI is rather bothersome. So, an automatic method was put forward. Firstly, data from IQI was cut to segments by certain width from beginning to end, and each segment was normalized. All the segments were discriminated according to gray values of beginning position and end position, and qualified segments need to be processed further. Secondly, dip position in duplex wire curve was searched according to an algorithm, and its position value in the segmented curve can be used to determine the modulation. The proposed method is able to determine modulation of each duplex wire quickly and reliably through processing just a time.