Technical Abstract: Erosion models play an important role in assessing the influence of human activities on the environment. For cold areas, adequate frost simulation is crucial for predicting surface runoff and water erosion. The Water Erosion Prediction Project (WEPP) model, physically-based erosion-prediction software developed by the USDA, has a major component to simulate winter processes, including snow accumulation and melt as well as soil freeze and thaw. WEPP is successfully used in the evaluation of important natural resource issues throughout the US and in a number of other countries. However, a preliminary study revealed problems in the winter component of the WEPP model, especially the routine for frost simulation. The main purpose of this study was to improve the WEPP model v2006.5 so that it can more adequately simulate soil freeze-thaw and winter runoff and erosion. The specific objectives were: (1) to modify the related algorithms and subroutines in WEPP v2006.5 that improperly represent soil freeze-thaw processes; and (2) to assess the performance of the modified model by applying it to two research plots under different climatic conditions. Changes in soil profile discretization and computation of key thermal and hydraulic parameters were made in the frost simulation routines of WEPP (v2006.5), and the modified WEPP (v2008.9) was applied to experimental plots at Pullman, WA, and Morris, MN. Simulation results from WEPP v2008.9 were compared with those from WEPP v2006.5 as well as with field observations. The results from WEPP v2008.9 showed substantially improved agreement with field data compared to those from WEPP v 2006.5.