Abstract: Propagation characteristics of longitudinal modes of ultrasonic guided waves are investigated theoretically and experimentally for the purpose of defect detection and stress measurement in steel strands. The characteristics,such as dispersion and multi-mode,of longitudinal modes for different kinds of wires in steel strands are analyzed theoretically.In the experiment,longitudinal modes are excited and received in a 15 meter long,17.80 mm nominal diameter steel strand and its helical wire and center wire in the experiment under the theoretical guidance.When longitudinal modes excited in free steel strands,their main energy propagates in helical wires.At partial frequencies in the frequency range of 0~400 kHz,experimental propagation velocities of L (0,1) mode,the lowest longitudinal mode,in the steel strand and its helical wire and center wire are well agreement with theoretical prediction.