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,　17.80mm　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　0-400kHz,　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.