Fabrication of microstructures via the microcasting technique involves two consecutive steps, namely, the capillary-driven flow of a nanoparticulate slurry into a sacrificial mold and burning of the organic binder phase followed by sintering of the resulting porous phase. During the filling process, settling of particulates may result in a particle concentration gradient in the mold. Consequently, the porous part before sintering has a spatial porosity distribution, which may dramatically influence the sintering kinetics and, in turn, the shape and dimensional fidelity of the final sintered product. This paper presents a theoretical model of the fluid flow during the microcasting of rectangular rib sections, with the goal of predicting the filling time and nanoparticulate distributions for various slurry systems and microchannel geometries. The model of the filling process is validated with previous experimental data reported by Morales et al. (2005), and is used in a parametric exploration of the effects of several nondimensional groups on the filling time and the particle distribution.