Abstract: The austenite grain growth tests for six heats of micro-alloyed high nitrogen L45 steels were experimented at the condition of long time heating (1800s) and stort time heating (5s) by rapid heating rate (50℃/s).The mean intercepts of austenite grains were measured. Utilizing the transmitted electron microscopic photoplates for extsacted replica the size distribution of the precipitated phases in steel 1^#, 3^# add 6^# were determined. The chemical composition of the fine precipitates was analysed by Energy Dispersion Spectrometer attached to the TEM. The mechanism of the effects of Al and the microalloyed elements Ti, V. Nh and of heating temperature on the austenite grain size was studied.