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Vegetation change can influence climate by altering the fluxes of mass and energy between ecosystems and the atmosphere. In the past century or so, rapid conversion of grasslands to woodland by woody species encroachment is one of the most important vegetation changes in the semiarid and arid regions of the world. The objective of this study is to investigate potential impacts of this widespread phenomenon on climate system in the southern Great Plains of the United States. The Regional Atmospheric Modeling System (RAMS) is used for this study. Grassland on the surface in RAMS is gradually replaced by woody species and RAMS is run a set of times, each with a different amount of encroachment. RAMS-simulated precipitation and air temperature are then analyzed. This study finds tha t over a 1year period woody plant encroachment leads to increase in rainfall and the increase is statistically significant at many locations. Woody encroachment also has an overall warming effect, but increase in temperature is not statistically significant. Temperature and precipitation increase almost linearly with increasing encroachment on the surface. When grassland is completely replaced, annual accumulated precipitation increases by 23.6mm and maximum air temperature rises by 0.13 degrees C averaged over the entire study area. In areas where encroachment occurs, averaged increases in accumulated precipitation and temperature are 58.2mm and 0.27 degrees C, respectively. The largest increase in precipitation and strongest warming tend to be located in dry and encroached areas including central and northern Texas, and they reach as high as 213.6mm and 0.68 degrees C, respectively. Decrease in surface albedo is found to be the most important factor that causes these changes.