A method for measuring distortion of a projection objective, which includes: obtaining a plurality of first positional deviations between two groups of patterns formed respectively after two exposures performed in a same exposure field during a stepping and exposing process of the reticle stage (S21); obtaining a plurality of second positional deviations between two groups of patterns formed respectively after another two exposures performed in a same exposure field during a stepping and exposing process of the workpiece stage (S22); subtracting motional errors of the reticle stage and/or workpiece stage from each of the plurality of first and second positional deviations to obtain corresponding first and second corrected deviations (S43, S44); calculating differences each between a pair of corrected deviations (S45); and calculating the distortion of the projection objective by a fitting process (S46).