Abstract: The basic configuration and imaging principle of high temperature confocal laser scanning microscopy (CLSM) were described. According to the characteristics of real-time observation and recording under high temperature, observation of in-situ structure of YF45MnV non-quenched and tempered steel and 12L14 free cutting steel was carried out. The results show that austenite grain changes of YF45MnV non-quenched and tempered steel at different temperatures and transition process from liquid phase→high temperature ferrite→austenite in 12L14 free cutting steel could be recorded clearly through CLSM. Austenite grain coarsening temperature of YF45MnV non-quenched and tempered steel was 1 220℃. In solidification process of 12L14 free cutting steel, austenite nucleated at the trigeminal grain boundary of ferrite phase, which initially was in sheet shape, and then developed rapidly in cellulation or thumb shape along the grain boundary of ferrite phase.