Shrinking of critical dimensions (CDs) in semiconductor circuits has been pushing optical lithography to print features smaller than the wavelength of light source. The demand for CD control is ever-increasing. In this paper, the study is conducted to reveal the impact of illumination pupil filling ellipticity on CD uniformity. As main parameters of CD uniformity, horizontal-vertical feature bias (H-V bias) and isolated-dense feature bias (I-D bias) caused by pupil filling ellipticity are calculated using the PROLITH software under four different illumination settings. Simulation shows that H-V bias and I-D bias are proportional to the pupil filling ellipticity. The slopes of the fitting lines of the H-V bias versus pupil filling ellipticity are calculated.