Abstract: Tensile test was performed on 11 Cr ferritic/martensitic steel at different temperatures (600-700 ℃) and strain rates of 2×10-5-1×10-3 s-1, and serrated flow during deformation was studied. The results show that "abnormal" serrated flow was found during tested steel deformation and only type E serrated wave exhibited. Serration behavior began to appear near the maximum tensile strength and serrated waves mainly lied in descending part of stress-strain curves. The effective activation energy for forming serration was 43.44 kJ·mol-1. At high temperature, the energy of substitutional atoms was high and could pin dislocations completely, and when a certain amount of dislocations up to a critical velocity and break free of pinning of substitutional atoms, the serrated flow appeared.