Abstract: The study focuses on fulfilling the on-line thermomechanical treatment(OLTM) process by intermediate cooling, reheating and reduction rolling of rough tubes after mandrel rolling. The process is beneficial to high qualities and lower costing for seamless tubes. It is found that the austentic grain grade by OLTM is smaller 2.5-3 ASTM than that by direct sizing, the properties of 40Mn2V steel by OLTM are the longitudinal toughness A_k≥23J,σ_0.5≥555Mpa, σ_b≥820Mpa, elongation δ₅≥23%, matching the property demands ofAPI Standard about N80 grade oil casing. The vital factors of improving impact toughness are the degree of austenite decomposition during intermediate cooling and the proper reheating temperature.