An　experimental　approach　for　determining　the　multimode　lamb　wave　velocity　of　a　thin　isotropic　aluminum　sheet　is　presented　in　this　paper.　Up　to　10　modes　are　finally　obtained　with　in　a　broadband　frequency　range.　A　lens-less　line-focus　transducer　whose　active　element　is　polyvinylidene　fluoride　(PVDF)　film　is　employed　by　the　defocusing　measurement　system.　The　V(f,　z)　analyzing　method　and　a　mode-tracing　algorithm　are　applied　in　the　waveform　processing　and　the　extraction　of　wave　velocity.　The　agreement　of　the　experimental　result　with　the　theoretical　value　indicates　that　the　dispersion　curve　can　be　determined　accurately.　The　discussions　on　the　future　applications　and　improvements　are　addressed.