In the most popular toner fusing process, toner melts are fused onto the paper under the influence of heat and pressure exerted by the nip forming rollers. The degree of toner penetration influences the toners fuse level and depends on process conditions, toner rheological and paper properties. This article establishes a three-dimensional (3D) model for studying toner penetration onto the paper macroscopically. The model considers a toner melt layer sandwiched between a smooth fuser roll and a porous paper surfaces under heat and pressure. Toner penetration and their thickness variation within the nip are computed under various conditions of toner viscosity, paper permeability, applied pressure, dwell time, and toner layer thickness. Toner viscosity is a strong function of temperature. Under normal fusing conditions and using Xerox 4024 plain paper, toner images just fill the voids at the paper surface. Only the less viscous liquid, such as fuser oil, can easily penetrate into the paper. Modeling results compare favorably with the actual toner images. The model can be applied to other penetration studies also.