Austenization is an important stage during the quenching-tempering heat-treatment process of X65 microalloyed pipeline steel, because it can influence the development of final microstructure and mechanical properties. In this paper, a detailed investigation was carried out on the austenizing transformation process in X65 microalloyed pipeline steel using high-resolution dilatometric technique and microstructure observations. According to the obtained dilatometric curve during continuous heating, the austenizing transformation process in X65 steel was apparently composed of two stages, 740-765 °C and 765-875 °C respectively. In order to clarify the microstructure evolution during the two stages, interrupt heat treatment tests were performed and subsequent microstructural observations showed that the first stage (740-765 °C) was corresponding to the dissolution of fine carbides particles and the second stage (765-875 °C) was corresponding to α→γ phase transformation. Firstly, austenite nucleates at interfaces between fine carbide particles and ferrite matrix due to the high interface energy there and then the carbide particles dissolve into the austenite nucleus, which constitutes the first stage. After the fine carbide particles dissolve completely into the austenite nucleus, the ferrite matrix relatively far from the original carbide particles needs higher thermal driving force to transform to austenite, therefore the major α→γ transformation occurs at higher temperature range (the second stage). © (2013) Trans Tech Publications, Switzerland.