Abstract: By using a high temperature microscope direct observation has been carried out in the TIG welded joint of GH99 nickel base superalloy at temperature 1200℃ under a certain stress and strain.The microstructure and dynamic process of initiation and propagation,of cracking were studied with a view to ascertaining the influence factors.The experimental results show that the principal factor affecting hot cracking is the chemical composition of alloy,especially the aluminium and titanium contents as well as Al/Ti ratio.The applied stress and strain condition can also affect the hot cracking to a certain extent.