Worldwide land use changes are being driven by the demand of food, fiber, water， and shelter to more than six billion people. Land use change is becoming an important anthropogenic forcing of global climate system as referred by IPCC AR4(IPCC,2007). The impacts of land use change on climate system can be divided into two major processes，that is biogeophysical and biogeochemical. The authors reviewed relative researches and hot topics on the two processes， respectively， and introduced two methodologies of assessing the relative importance for the two processes， one is to calculate their radiative forcing, the other is to use the couple model to simulate the two processes at the same time. These two methodologies were applied to access the policy of climate change such as afforestation, reforestation， and carbon sequestration. The uncertainties and further research topics were also discussed.