The impact of Non-ideal illumination pupil on imaging performance of lithography is calculated using software PROLITH. Effect of the sensitivity of image CD placement error caused by a shift of illumination with numerical aperture (NA) and coherent factor (σ) is also calculated. Calculations show that the main effect of a shift of illumination on imaging performance of lithography is the image CD placement error in the ideal optical systems. The image CD placement error caused by a shift of illumination is proportional to the defocus, and the slope of line increases with increasing the shift of illumination. A shift of illumination has little effect on H-V bias and Depth of Focus (DOF). The impact of a shift of illumination and pupil filling unbalance on image CD placement error is the sum of the CD placement error induced separately.