The impacts of pupil filling unbalance on image CD placement error, H-V bias (the size difference between the vertical lines and the horizontal lines) and I-D bias (the size difference between the isolated lines and the dense lines) are calculated using software PROLITH under annular and quardupole illumination settings. Calculations show that the main effect of pupil filling unbalance on imaging performance of lithography is the image CD placement error. The image CD placement error caused by pupil filling unbalance is proportional to the pupil filling unbalance. The tolerance of the pupil filling unbalance of 5% is achieved according to the budget of the overlay accuracy.