Abstract: A low carbon steel(with Ni addition) with low temperature(-100℃) impact resistance is selected to analyze how 4 kinds of treament processes affect the mechanical properties and metallographic structure of the seamless steel pipe made by it, i.e.hot rolling+hot expanding, hot rolling+quenching& tempering, hot rolling+quenching& tempering+hot expanding, and hot rolling+hot expanding+quenching& tempering.The analysis leads to the following conclusions.Comparing to other processes, the hot-rolled and hot-expanded product has lower tensile strength and its-100℃ impact energy is almost 0.For hot-expanded pipe with a big diameter change, although the mother pipe before expanding has good properties after quenching& tempering, the properties of the expanded seamless steel pipe are decreased, especially its low temperature impact toughness.And when diameter change is not big, the low carbon manganese steel/low alloy steel mother pipes, after quenching& tempering, can be directly hot-expanded to big diameter thin wall seamless steel pipes.