The greenhouse gas (GHG) emission of reservoirs and its effects on climate change not only are widely concerned by public, but also have important influence on the future development of hydro power and reservoir construction. In this paper the reservoir features of the main areas in the world are compared based on the research results at home and abroad from power density and reservoir area per unit generating capacity. The influences of GHG emissions of reservoir are studied with analyzing the reservoir features, reservoir operation ways and their geographic environments. Finally, the GHG emission of the Three Gorges Reservoir is discussed. It is concluded that the hydro power has obvious advantage in reducing GHG emission than thermo power generation. Most reservoirs in the world, including Three Gorges Reservoir, have very little GHG emission and effect on climate change.