A method of calibrating a test chart is provided. First, a reference scanning device scans a reference test chart to obtain a plurality of reference optical density (OD) values. The reference test chart includes a plurality of reference blocks. Then, the reference scanning device scans a test chart to obtain a plurality of first OD values. The test chart includes a plurality of blocks, which corresponds to the reference blocks. Next, a compensation function derived from respectively converting the first OD values into the reference OD values is obtained.