The microstructure evolutions of Ni–Cr–W based superalloy during thermal exposure have been investigated systematically. M 6 C carbides in the alloy decompose into M 23 C 6 carbides at temperatures from 650 to 1000°C due to its high content of Cr. The M 6 C carbides decompose dramatically from 800 to 900°C. At temperatures up to 1000°C, a few M 23 C 6 carbides form on the surface of M 6 C carbides. The decomposition behavior of primary M 6 C can be explained by the following reaction: M 6 C→M 23 C 6 +Me (W, Ni, Cr, Mo). At temperatures below 900°C, coarse lamellar M 23 C 6 carbides precipitate at the grain boundaries. The carbide lamellae line almost perpendicular to the grain boundaries. While the temperature is above 1000°C, discrete M 23 C 6 carbides precipitate at the grain boundaries. Moreover, there are lots of small M 23 C 6 particles precipitated around M 6 C carbides from 650 to 1000°C.