Abstract: The human eye is the best optical device in the world. The performance of this eye is influenced by optical aberrations. Several human eye models were developed to treat and correct this issue. This work depends on the Liou and Brennan eye model to design and analysis the perfect and an aberrated human eye. This model is more suitable than other models, since, it depends on using a gradient refractive index (n) with aspherical lens surface. The effect of entrance pupil diameter on optical aberrations was evaluated by programmatically by ZEMAX, computer software (Version 14) at 555 nm monochromatic light under the influence of Semi-Diameter values Variation by 1.25, 1.5, 2, 3, 4 and 5 mm. The Modulation Transfer Function (MTF), wave front aberration and Root Mean Square of the spot sizes (RMS) were used to analysis the performance of the eye. The results show that the best image quality obtained at 1.25 and 1.5 mm Semi-diameter which mean the lowest values of aberrations can be achieved by smaller pupil size and small field of view under monochromatic light.