For any plant, it is very important to generate electricity without forced outages. Failure of boiler tubes is the major concern of forced outages at the coalfired thermal power plant, especially at ultra-supercritical units. 12Cr1MoV steel has been widely used as tubes in the ultra-supercritical boiler. Cause of the failed reheater tube joint of 12Cr1MoVG steel was studied by chemical composition test, microstructure analysis, crack morphology observation，joint structure, hardness, and welding qualities analysis. The results show that the crack initiated from coarse grain heat affect zone (HAZ) along with the circumferential distribution, and propagated along grain boundaries. There are macrocracks，microcracks and voids in the cracking region．According to the microstructure，position，process and fractography analyses of the cracks，it is concluded to be the weld reheat cracking. The failure of the tube is mainly due to bad welding quality, characterized by high residual stress, coarsened grain size, and so on. Formation of intergranular cavity and microcrack is shortened by high residual stress and is easier due to plastic-deformation disharmony between coarse grains.