Longitudinal　ultrasonic　guided　waves　propagation　in　fluid-filled　cylindrical　steel　pipe　is　investigated　numerically　and　experimentally.　By　using　displacement　and　stress　boundary　conditions,　dispersion　equation　for　longitudinal　guided　waves　in　fluid-filled　steel　pipe　with　quiescent　or　flowing　fluid　is　derived　and　numerical　analyzed.　According　to　numerical　results,　it　is　shown　that　optimized　guided　waves　have　potentials　to　measure　flow　rate　of　fluid　filled　in　narrow　cylindrical　pipes.　In　experiments,　the　4　PZT　wafers　adhered　to　one　end　of　the　outer　surface　of　a　steel　pipe　filled　with　and　without　water　are　used　as　the　actuators　and　receivers　simultaneously.　According　to　received　signals,　experimental　propagation　velocities　of　longitudinal　guided　wave　modes　are　obtained.　Experimental　results　show　a　good　agreement　with　theoretical　predication.　Foundational　experimental　results　show　that　longitudinal　ultrasonic　guided　waves　can　be　used　for　flow　rate　measurement　of　fluid　in　narrow　pipes.　©　2010　IEEE.