Stress　loss　in　steel　strands　will　threaten　the　safety　of　whole　prestressed　structures.　Therefore,　it　is　important　to　achieve　in-service　prestress　measurement　nondestructively　of　steel　strand　in　engineering.　Prestress　measurement　method　in　steel　strand　based　on　ultrasonic　guided　wave　technique　is　analyzed　theoretically　and　verified　in　the　experiment.　According　to　wave　theory　and　acoustoelastic　theory,　longitudinal　and　shear　wave　velocities　in　steel　strand　under　different　applied　prestress　levels　are　obtained,　the　relationship　between　group　velocity　of　the　lowest　longitudinal　guided　mode　L(0,　1)　and　applied　stress　in　steel　strand　is　analyzed　and　constructed.　In　experiment,　L(0,　1)　mode　of　ultrasonic　guided　waves　is　excited　and　received　by　a　pair　of　self-developed　magnetostrictive　sensors　in　a　seven-wire,　17.80　mm　nominal　diameter　steel　strand　under　different　applied　prestress　levels.　Group　velocities　of　L(0,　1)　mode　under　different　stress　state　conditions　are　measured　in　the　steel　strand　and　the　actual　relation　curve　between　experimental　group　velocities　of　the　mode　and　applied　stresses　in　the　steel　strand　is　obtained.　Results　show　that　group　velocity　of　L(0,　1)　mode　will　decrease　approximately　linearly　with　the　increase　of　applied　stress　when　the　stress　is　above　0.5　GPa.　Experimental　results　are　well　coincident　with　predicted　assumption.　The　conclusion　provides　a　certain　experimental　foundation　for　application　of　ultrasonic　guided　waves　technique　to　measurement　of　applied　prestress　in　steel　strand.　©2010　Journal　of　Mechanical　Engineering.