A computational model of the image formation process has been developed for the Nomarski differential interference contrast (DIC) microscope. The DIC microscope images variations of the phase of the light wave transmitted through the specimen. In the study of biological phenomena, the DIC microscope is used to visualize live cells which are highly transparent in the visible spectra but distort the phase of the impinging light wave. Within the microscope, a birefringent prism splits the transmitted light wave into two laterally sheared wavefronts. An interference pattern is imaged when the wavefronts recombine. The computational model we developed uses polarization ray-tracing techniques. Rays propagating through different microscope components and the specimen are traced. A specimen is represented by a 3D grid of voxels, each containing a complex refractive index. At the image plane, a coherent superposition of the diffracted field due to each ray contributes to the image intensity. Partial coherence at the image plane is also simulated by wavefronts with different propagation directions. By computing the image intensity at different positions along the axial direction, we can obtain optically sectioned images. In order to evaluate our model, we compared simulated images to the images taken under a real DIC microscope. We constructed test specimens of known shape and properties, using polystyrene beads in optical cement and an etched glass wafer. As the next step, we plan to use this computational model for the reverse problem, i.e. to reconstruct the 3D refractive index distribution of an imaged specimen.