Abstract: The 07MnNiMoVDR steel for low-temperature pressure vessel was investigated via thermal expansion testing in combination with TEM detection for the transformation of microstructure between different temperature intervals and change of carbide precipitation in the tempering process. The results showed that when the specimens are tempered at 300°C the residual austenite is decomposed and the lamellar θ-carbides are found between α-phase laths which are still of relatively high dislocation density. When tempered at 650°C, the α-phase laths are recovered with part of them recrystallized. Moreover, the precipitates are distributed between the recovered α-phase laths and on the recrystallized grain boundaries.