Several numerical codes were developed and used to evaluate the erosion thickness of the plasma facing wall. Some discrepancies of the results using the existing codes are pointed out. It is very important to understand the reason of the discrepancies. The analytical studies were performed to find the sensitivity of some factors on the results, such as boundary conditions, evaporation models and thermal properties. It was found that the temperature dependency effects of thermal conductivity and vapor pressure strongly affected the erosion results. Unfortunately, the results of this study cannot sufficiently explain the discrepancies. We would like to emphasize the necessity of the verification of the existing codes and numerical models, and the physical understanding.