Chemical mechanical polishing (CMP) is a widely used technique to achieve high level of global and local planarity required in integrate circuit (IC) areas, which pleas for concentrate researches. A preliminary wafer-scale flow model for CMP is presented considering the roughness as well as the porosity and compressibility of the pad. Pressure distributions for three kinds of pad roughness: cosine shape, two-scale cosine shape and actual roughness were given with the help of numerical simulation by solving the corresponding two-dimensional slurry flow model. Pressure fluctuations and peaks can be seen from the results. The model predictions will be conducive to the removal rate and mass transport computation. The research is a qualitative one and will pave the way for further explorations of mechanisms of CMP process.