The　guardrail　is　one　of　the　most　important　attachments　in　the　highway.　The　compactness　of　the　soil　around　it,　called　as　embedded　quality,　determines　the　crashworthiness　performance　of　the　guardrails.　Unqualified　guardrail　posts　can　not　avoid　severe　accidents　happen.　But　there　are　not　efficient　nondestructive　detecting　methods　until　now.　So　we　research　on　the　applications　of　ultrasonic　guided　waves　technology　to　inspect　guardrail　post　length　and　embedded　quality.　For　cylindrical　guardrail　posts,　ultrasonic　guided　waves　can　be　used　for　their　length　measurement　and　evaluation　of　embedded　quality.　In　this　paper,　the　effect　of　the　parameters　of　soil　including　density,　the　velocities　of　longitudinal　and　shear　waves　on　propagation　characteristics　of　longitudinal　guided　waves　modes　is　analyzed　and　verified　experimentally.　It　indicates　the　change　of　soil　characteristics　have　no　effect　on　group　velocity　of　L(0,2)　mode　in　the　frequency　range　from　160kHz　to　270kHz　and　it　will　convenient　to　the　length　measurement　of　guardrail　post　with　different　embedded　conditions.　However,　the　attenuation　of　the　same　L(0,2)　mode　increases　monotonously　with　increasing　soil　compactness.　As　a　result,　L(0,2)　mode　in　the　frequency　range　from　160kHz　to　270kHz　can　be　used　to　evaluate　embedded　quality　of　guardrail　post　to　improve　the　safety　of　guardrails　in　the　highway.　©　2009　SPIE.