In　order　to　study　the　transmitting　characteristics　of　ultrasonic　guided　wave　in　partly　embedded　guardrail　post　and　select　the　suitable　guided　waves　mode　and　frequency　for　inspecting　post　length,　authors　calculated　and　analyzed　the　guide　wave　group　velocities,　attenuation　and　dispersion　in　unembedded　part　and　buried　part　of　the　posts　respectively　on　the　basis　of　elastic　dynamics　theory.　Results　show　that　the　group　velocities　of　longitudinal　mode　L　(0,　2)　are　the　same　in　the　frequency　range　of　80　kHz　to　130　kHz　for　the　unembedded　part　and　embedded　part　of　the　posts.　L　(0,　2)　mode　can　propagate　enough　long　distance　in　the　post　so　that　it　can　be　used　to　measure　the　length　of　posts　partly　embedded　in　soil　because　its　attenuation　is　the　lowest　in　all　modes.　At　the　same　time,　the　experiment　result　is　consistent　with　results　of　theoretical　analysis.　Waveform　of　end　echo　is　clear　and　has　high　signal-to-noise　ratio.　The　maximal　relative　error　is　less　than　0.8%.　This　method　can　meet　the　needs　of　engineering　application.