In the lateral shearing interferometer for EUVL PO at-wavelength measurement, a grating is used to separate and shear the test wavefront. Normally, the astigmatism component of the test wavefront is difficult to be measured from the phase distribution tilt because the grating position influences it also. We found the grating position influences the tilt component of the interferogram phase distribution along the shear direction only. A part of the astigmatism can be measured from the tilt component perpendicular to the shear direction. To measure the other part of astigmatism, we perform a measurement with the shear direction in 45degrees. From the tilt component of the phase distribution in 135degrees, the other part of the astigmatism can be measured. By these two measurements, the whole astigmatism can be measured. We analyzed the measurement errors and showed the conditions for high accuracy astigmatism measurement. (C) 2004 American Vacuum Society.