Abstract: Thermal simulation technique is adopted to investigate the microstructure and properties of over-heated zone and fine grained zone of 10Ni8CrMoV steel.The results show that the toughness of over-heated zone increased greatly because of austenite film lying in lathy martensite and a few ferrite.The toughness of fine grained zone decreased due to many factors, such as acicular martensite, a small quantity of austenite film, carbide separation and so on.Then fine grained zone might become the weak zone in welded joint.