Flow, mixing, and, size segregation of Municipal Solid Waste (MSW) particles on the traveling grate of a mass-burn waste-to-energy (WTE) combustion chamber is analyzed for understanding those parameters that control the combustion processes and for designing the chamber. In order to quantify these phenomena, a full-scale physical model of the reverse acting grate was built and used for investigating the effects of the motion of the reverse acting grate under a MSW packed bed with tracer particles ranging from 6 – 22 cm in diameter. Based on these experimental data, a stochastic model of MSW particle within the packed bed on a traveling grate was applied for simulating the MSW particle behavior. The result shows that the motion of the traveling grate, whose speed ranged from 15 to 90 reciprocations/hour, increases the mean residence time of small and medium particles by 68% and 8%, respectively, while decreasing the mean residence time of large particles by 17%. This is because of size segregation of particles known as the Brazil Nut Effect. When the ratio of particle diameter to the height of moving bar, d/h, increases from 0.46 to 1.69, the mixing diffusion coefficient, De at 60/hour., decreases from 96 to 38.4. This indicates that the height of the moving bars should be greater than the diameter of targeted particles.