Precisely evaluating the geometrical attenuation factor is critical for constructing a more complete bidirectional reflectance distribution function (BRDF) model. Conventional theories for determining the geometrical attenuation factor neglect the correlation between height and slope and the self-shadowing or self-masking effects on microsurfaces, leading to results that are discrepant from reality, apparently. This paper presents a three-dimensional (3D) geometrical attenuation factor formulation on 3D Gaussian random rough surfaces. The proposed numerical analysis of 3D geometrical attenuation factor is much more precise for a practical application, especially near grazing angles. Our proposed numerical analysis of 3D geometrical attenuation factor can precisely evaluate the BRDF model.