Abstract: The microstructure and properties of X80 steel will be changed greatly because of heat cycle in welding process.Toughness is a very important property of long distance natural gas transmission pipeline.The toughness characteristic(Charpy-V impact and crack opening displacement,i.e.CTOD)and reason of X80 Coarse Grained Heat Affect Zone(CGHAZ) with different welding heat inputs was analyzed by weld thermal simulation technique,modern test and microstructure analyses methods.The results indicted that the toughness of CGHAZ of higher heat input is better than that of lower heat input in some extend,and the toughness violently decrease as increasing of heat input beyond the extend.The essential reason of these results is the different microstructures of CGHAZ for different heat input.The low carbon martensite constituent found for lower heat input makes CGHAZ toughness be bad.