Hot-corrosion directionally solidified Nickel base superalloy DZ444 is generally used as the candidate material for blade of gas turbine, which required excellent alloy microstructural stability. As one of the constitutional phase of the alloy, primary MC carbides are often thermally unstable, and its degradation reactions can happen when the alloys are in services or thermally exposed in high temperature circumstances. There existed several different kinds of MC decompostion reactions in some traditional Ni-based superalloys. To figure out the thermal stability of primary MC carbide in the DZ444 alloy and better understand its degradation mechanism, some related discussions to the thermal stability and degeneration process of primary MC carbide and its effects on the microstructure were made. In this work, microstructures of DZ444 alloy after long-term exposure up to 1×10⁴ h at 800, 850 and 900 ℃ have been observed by OM, SEM and TEM. The results show that the thermal stability of MC was low. As long-term exposure proceeds, MC decompostion became more and more serious. Firstly, a typical sandwich microstructure (SM) gradually formed and thickened in the MC/g interface; secondly, h phase precipitated in the SM/MC interface; lastly, h-M₆C and h-M₂₃C₆ locally precipitated inside the h phase. Finally, SM structure, h phase, h-M₆C and h-M₂₃C₆ successively formed in MC degeneration areas at three stages of its decomposition process. Basically, MC decompostion process could be described with such raction formula as follows: MC+g→SM-M₂₃C₆+SM-M₆C+SM-g'→SM-M₂₃C₆+SM-M₆C+SM-g'+h→SM-M₂₃C₆+SM-M₆C+SM-g'+h+h-M₆C+h-M₂₃C₆. Generally, the type of secondary carbide from MC degeneration was M₂₃C₆, and, with the increase of long-term exposure temperature and time, the amount of secondary M₆C carbide slightly increased. Besides, MC degeneration might result in the precipitation of transgranular M₂₃C₆ carbide and s phase in the vicinity of MC degeneration areas, and the coarsening of grain boundary (GB).