A numerical model was developed to investigate the influence of gravitational force on the transport of colloids in a single horizontal fracture-matrix system. Along with major transport phenomena, prominence was given to study the mass flux at the fracture-matrix interface, and colloid penetration within the rock matrix. Results suggest that the gravitational force significantly alters and controls the velocity of colloids in the fracture. Further, it was shown that the colloid density and size play a vital part in determining the extent that gravity may influence the transport of colloids in both fracture and rock matrix. The mass flux transfer across the fracture-matrix interface is predominantly dependent on the colloidal size. As large as 80% reduction in penetration of colloids in the rock matrix was observed when the size of the colloid was increased from 50-600 nm. Similarly, the farther the density of colloid from that of the fluid in the fracture (water), then the higher the mitigation of colloids in the fracture and the rock matrix. Finally, a non-dimensional parameter "Rock Saturation Factor" has been presented in the present study, which can offer a straightforward approach for evaluating the extent of penetration of colloids within the rock matrix. © 2018 by the authors.