A　low-frequency　ultrasonic　method　presented　in　this　paper　is　suitable　for　measuring　any　one　of　the　four　characteristic　parameters,　i.e.,　thickness,　density,　shear,　and　longitudinal　wave　velocities,　of　a　thin　elastic　layer　given　the　remaining　three.　Thickness　of　the　thin　layer　is　much　smaller　than　the　longitudinal　wavelength　of　the　ultrasonic　wave　used　in　the　experiments.　The　method　employs　the　water　immersion　pitch-catch　technique,　and　is　based　on　the　dispersion　properties　of　the　first-order　symmetric　and　anti-symmetric　modes　of　a　Lamb　waveguide.　The　unknown　parameter　is　estimated　by　minimizing　the　mean　square　error　obtained　by　comparing　theoretical　dispersion　curves　with　experimental　data.　A　wide-band　excitation　technique　is　used　to　reduce　measurement　time,　by　which　a　frequency　range　of　possible　guided　Lamb　waves　is　simultaneously　explored.　The　sensitivity　and　accuracy　of　the　proposed　technique　for　different　parameters　in　different　thicknesses　are　analyzed.　Using　the　present　technique,　a　thin　layer　with　thickness　down　to　one　percent　of　the　longitudinal　wavelength　can　be　successfully　measured　with　an　average　error　of　about　two-percent.　But　the　method　fails　to　evaluate　the　density　of　the　thin　layer　if　the　thickness　is　less　than　five　percent　of　longitudinal　wavelength.