In this paper, the evolution of precipitate phases in 9CrMoCoB (CB2) steel after tempering at 650 °C for different times has been investigated using transmission electron microscopy. As the tempering time prolonged from 0 to 100 h, four types of secondary phases consisting of M 3 C, M 2 X, M 7 C 3 and M 23 C 6 were identified in the experimental steel. It was found that the needle-like M 3 C carbides could be precipitated during both the air cooling process after austenization and the early stage of tempering. After tempering for 15 to 120 min, fine M 2 X particles were observed. The precipitation and growth of M 2 X were derived from the partially consumption of M 3 C carbides. After tempering for 50 h, M 2 X disappeared, while the rod-like M 7 C 3 carbides were observed to be precipitated independent of the early formed M 3 C precipitates. After tempering for 100 h, only M 23 C 6 carbides persisted, of which the number density and size kept increasing throughout 30 min to 100 h owing to their high thermostability. In addition, the precipitation mechanisms of the four types of precipitates were explained, and the relationship between the measured hardness values and microstructure was discussed.