Abstract: The bainitic transformation behavior of the 25MnV steel is studied with the expansimeter method and under two experimental process conditions as simulating rolling and rolling-control & cooling-control process conditions for seamless steel tube manufacture. The result shows that with higher rolling-end temperature,after getting the material quickly cooled down to 390~485 ℃,and then subjected to slow cooling,full bainitic structure with hardness higher than 22HRC is formed,which leads to such a conclusion that it is possible to produce the bainitic N80 steel oil casing with the rolling-control & cooling-control process. The research achievement provides basis for development of the non-quenched-and-tempered bainitic steel tube.