Abstract:  This paper studies austenite transformation products of 38MnVTi non-quenched and tempered steel during isothermal cooling and the relations between transformation amount and time. The expansion curves of different isothermal temperatures are obtained by Gleeble-1 500D thermal simulation tester. The expansion curves at different isothermal temperatures are acquired by expansion method on Gleeble-1 500D thermal simulation testing machine. The isothermal transformation curve (TTT curve) of 38MnVTi non-quenched and tempered steel is calculated and fitted by Origin software. Our results show the precipitation temperature of cementite is 750 ℃, the ferrite precipitation temperature 840 ℃, the martensite precipitation temperature 360 ℃ and the martensite transformation termination temperature 212 ℃. Under the isothermal cooling conditions of 540, 510, 480 and 430 ℃, the maximum transformation amounts of bainite are 16.9%, 26.8%, 38.5% and 55.3%, respectively.